The reason why is because kids are back in school (if they are off). School holidays tend to pack the FL parks a bit, especially IOA since there is a school right outside of Dragon Challenge (and no, not Hogwarts!)
Its the theme park that the Wizarding World of Harry Potter is in!
Like I said, Tuesday because the kids are likely in school.
Walt Disney World's Magic Kingdom
Disney's Animal Kingdom
Disney's Hollywood Studios
Disney California Adventure
Universal's Islands of Adventure
Universal Studios Florida
Universal Studios Hollywood