Quoted By:
Because it's only really celebrated in America, and animation is a worldwide medium. Unless being American is part of the show's identity (e.g. The Simpsons, KOTH, Rugrats, etc.), celebrating a holiday most countries aren't familiar with wouldn't make much sense. It's not like with Christmas or Halloween where, while not celebrated by every nation, is still a holiday familiar to most first world countries and markets these cartoons are aimed towards.
It's also the same reason there aren't many Easter or Hanukkah specials. They're very religious specific holidays.
