KA Applegate did a Reddit IAMA session in 2001. The implication seems to be that her decision to stop writing the Everworld novels was driven by dramatically declining sales figures compared to the large commitment required to generate new novels.
Q. The Everworld series was probably my favorite thing to read as a teenager. It seems like it would be the perfect material for a movie
and or series. Has there been any interest shown in that regard?
katherineapplegate: Thanks. You and six other people. No Hollywood love yet. It's all complicated and controlled by
Q. Was the series not popular? I figured the blend of mythology, magic and aliens would appeal to a large audience.
katherineapplegate: No, it died a painful (and not very slow) death.
IAm K.A. Applegate, author of Animorphs and many other books. AMA
Note that I've edited slightly to remove chatter.
As regards the final book (terminating the series) it would appear that the author wasn't especially pleased with it either.
Q. Are you satisfied with how EverWorld ended? It wasn't very satisfactory as a reader. Can you explain why you ended it the way you
katherineapplegate: I'm not happy with the EW ending. Basically I overcommitted. We could keep up with 140 pages a month -- barely --
but Everworld was 250. We got in over our heads.
Additionally, there seems to have been a certain amount of burnout on the part of the author
katherineapplegate: In an exhausted, burned out wheeze. By the 4th Remnants and the next
to last Everworld, we were burned out. Really done for. 150 books
between 1989 and whatever it was, 2000. It was nuts and we had to
stop. We didn't write at all for about 5 years.