Probably because of his perfectionism in the art he creates.
Extensive interview quotes follow. Skip to the end for a summary.
In an
interview from 1989, when
Calvin & Hobbes was still going strong, Watterson explains that he was never aiming to become hugely successful or popular, that he writes within firm self-imposed limits which he doesn't want to leave, even if that makes his writing more difficult.
Well, in the last three years, have the fantasy sequences gotten easier? You seem to be doing less of them these days. Is there a reason?
WATTERSON: At first it was fun simply to juxtapose fantasy with reality [...] That was originally a fun device, but the burden on the strip has been to make each switch more clever. The juxtaposition alone can get predictable if it’s just done over and over in the same way. Each time it’s got to be done with some unpredictability: some cleverness to it so that it doesn’t become moribund. So, yes, I’m doing fewer because it’s getting more and more difficult. But I still try to do the fantasies as they interest me. There’s a limitation to them. They’re fun to read and they’re certainly fun to draw, but they don’t have the emotional weight to them that an interaction between two interesting characters does.
In the same interview, Watterson seemed very cynical about the idea of licensing his work to other forms of media, claiming that this would degrade quality to an extent which wouldn't be acceptable to him, the creator.
You’ve rejected licensing your strip’s characters. Why?
WATTERSON: Basically, I’ve decided that licensing is inconsistent with what I’m trying to do with Calvin and Hobbes. I take cartoons seriously as an art form, so I think with an issue like licensing, it’s important to analyze what my strip is about, and what makes it work. [...] I’m not interested in removing all the subtlety from my work to condense it for a product. The strip is about more than jokes. I think the syndicate would admit this if they would start looking at my strip instead of just the royalty checks. Unfortunately, they are in the cartoon business only because it makes money, so arguments about artistic intentions are never very persuasive to them. I have no aversion to obscene wealth, but that’s not my motivation either. I think to license Calvin and Hobbes would ruin the most precious qualities of my strip and, once that happens, you can’t buy those qualities back.
I’m sure some of the readers will say to all this, “Come on. The comic strip is a popular art form.” What’s wrong with indulging the public’s interest?
WATTERSON: Nothing, so long as it doesn’t compromise the art itself. In my case, I’m convinced that licensing would sell out the soul of Calvin and Hobbes. The world of a comic strip is much more fragile than most people realize. Once you’ve given up its integrity, that’s it. I want to make sure that never happens. Instead of asking what’s wrong with rampant commercialism, we ought to be asking, “What justifies it?” Popular art does not have to pander to the lowest level of intelligence and taste.
All of this points towards a man who, when he became unable to continue his comic strip with a standard of quality that he felt was acceptable, would point-blank refuse to let it be continued at all. Neither to carry on himself, continuing to make money but allowing his standards to fall, nor to permit anyone else to continue in his place. This is also touched upon in another interview (date unknown, but still during Calvin & Hobbes's heyday):
Do you see yourself doing this forever?
WATTERSON: I'd like to, yeah, if the market will bear it.
Calvin and Hobbes exclusively?
WATTERSON: Yeah, I'm really enjoying the work. I feel that the characters have a lot of potential. I'd like to have the opportunity to draw this strip for years and see where it goes. It's sort of a scary thing now to imagine; these cartoonists who've been drawing a strip for twenty years. I can't imagine coming up with that much material. If I just take it day by day, though, it's a lot of fun, and I do think I have a long way to go before I've exhausted the possibilities.
Do you think you'll ever need a ghost?
WATTERSON: No, that's against what I believe about comic strips. In fact, I'd go even further and say I don't think a strip should ever be continued after the death or retirement of a cartoonist.
Well, you know, a lot of the very good ones used assistants.
WATTERSON: Yeah, Pogo did. Schulz has a good comment on that: "It's like Arnold Palmer having someone to hit his chip shots." I spent five years trying to get this stupid job and now that I have it I'm not going to hire it out to somebody else. The whole pleasure for me is having the opportunity to do a comic strip for a living, and now that I've finally got that I'm not going to give it away. It also gives me complete creative control. Any time somebody else has their hand in the ink it's changing the product, and I enjoy the responsibility for this product. I'm willing to take the blame if the strip goes down the drain, and I want the credit if it succeeds. So long as it has my name on it, I want it to be mine. I don't know, if you don't have that kind of investment in it...I guess that's the difference between looking at it as an art and looking at it as a job. I'm not interested in setting up an assembly line to produce this thing more efficiently. There are certainly people who could letter the strip better than I do; I don't enjoy lettering very much, but that's the way I write and that belongs in the strip because the strip is a reflection of me. If cartoonists would look at this more as an art than as a part time job or a get-rich-quick scheme, I think comics overall would be better. I think there's a tremendous potential to be tapped.
Admittedly this is from long before he quit writing Calvin & Hobbes, but when asked whether he'd stick with that strip exclusively, his answer was yes. This may be part of the reason why he hasn't continued writing since: he preferred to have his name associated with one successful product than risk going downhill in the future for the sake of cashing in.
I also found a much more recent interview (2013) where Watterson talks about what he's done since finishing Calvin & Hobbes. This is perhaps the most relevant in answering your question.
There is a tendency to rehash and regurgitate properties with sequels and remakes. You had an idea, executed it, then moved on. And you ignored the clamor for more. Why is it so hard for readers to let go?
WATTERSON: Well, coming at a new work requires a certain amount of patience and energy, and there’s always the risk of disappointment. You can’t really blame people for preferring more of what they already know and like. The trade-off, of course, is that predictability is boring. Repetition is the death of magic.
Although he hasn't continued writing since Calvin & Hobbes, he's still been creating art, in the form of paintings. He explained that he doesn't display this because he can't handle the level of publicity and fame that Calvin & Hobbes gave him.
According to your collection introductions, you took up painting after the strip ended. Why don’t you exhibit the work?
My first problem is that I don’t paint ambitiously. It’s all catch and release—just tiny fish that aren’t really worth the trouble to clean and cook. But yes, my second problem is that Calvin and Hobbes created a level of attention and expectation that I don't know how to process.
(Bold emphasis mine in all the above quotes.)
There is at least one source which may contain more info to answer your question:
the book Exploring Calvin and Hobbes, which contains far more lengthy interviews with Watterson than any of what I've quoted above. But I haven't found a copy of this book, and the interviews above seem to paint a reasonably clear picture already of the answer to your question:
- he didn't continue writing because he didn't feel he could keep on satisfying his own standards of quality, and refused to work at a lower standard;
- he didn't expand his universe because he felt that would compromise the original art;
- he didn't stay in the public eye because he was uncomfortable with the attention.
