It's because they have two different uses (three if you count nested quotes, but those aren't common and are pretty easy to figure out), contractions and possession, and they seemingly collide on words like "its" where you'd think it could mean either.

Not sure if you've already learned this (or if it helps), but English used to be declined, and its pronouns still are, e.g. they/their/them. That's why "its" isn't contracted; the possessive marker is already in the word.