Nothing complicated here. The Cat (in the hat) is the star—the hero. Everything about him screams, "Name this book after me! Put me on the cover!" Whether you know him as El Gato en el Sombrero, Le Chat Au Chapeau, Chatul Taalul, or even Cattus Petasatus, this guy wears his name well.
We're just going to lay it out there: the title The Cat in the Hat sounds funny. Right? It sounds like the cat is inside the hat. Wouldn't a more accurate way of putting it be The Hat on the Cat?
That's just limited thinking, and boring to boot. Just what Seuss wanted to avoid. Right down to the title, this story encourages flexibility with language and effect over formal rules. Plus, if you're so set on hats, just check out The 500 Hats of Bartholomew Cubbins. That has all the hats you could ever hope for.
Writing with just a couple hundred words can be frustrating, but it's that frustration that gave Dr. Seuss his title. The Good Doctor once said:
I read the list of words three times and I almost went out of my head. I said, "I'll read it once more and if I can find two words that rhyme, that'll be the title of the book." I found 'cat' and 'hat' and I said, the title will be The Cat in the Hat. (Source.)
Not so much. His publisher isn't so sure this is true (source). Yes, Seuss was frustrated with the list, but he was already planning to have a cat for a star. And as the book itself shows, there were lots of words on the list that rhyme.
Oh well, we all like a good story.