Because things exist

(the antigravity hypothesis was an attempt to explain why matter and antimatter haven't annihilated each other)

Because one common assumption was that the universe might contain as much antimatter as matter.

Which begs the question: Where did it go? We would notice a huge amount of annihilation reactions in the solar system.

"Antimatter falls up" (is gravitationally repelled instead of attracted by normal matter) was an easy hypothesis to explain that.