The reason he wrote it is very interesting. He thought of the "person's a person, no matter how small" from his experiences in post-war Japan. He had drawn anti-Japanese cartoons during the war, and he realizes this: [from an interview here]
Well, Japan was just emerging, the people were voting for the first time, running their own lives—and the theme was obvious: “A person’s a person no matter how small,” though I don’t know how I ended up using elephants. And of course when the little boy stands up and yells “Yop!” and saves the whole place, that’s my statement about voting—everyone counts. It’s all left over from my war experience, when I was making propaganda and indoctrination films. One of them was to encourage soldiers to vote.
So the entire thing is an allegory for post-war Japan. The kangaroos, therefore, represents Americans and the Whos are the Japanese. The slogan "a person's a person, no matter how small" is really what Seuss is trying to get at.
On the other hand, he also said this on the same site:
Parenting: Do you use propagandistic skills in your books?
Seuss: of course. However, most of my books don’t carry heavy morals. The morals sneak in, as they do in all drama.