Why is it always the secondary character from an animated sitcom that end up becoming the most popular?

A main character in any show is pressured to be the be all, end all. That means he/she has to have all the necessary qualities, whether it'd be humor or otherwise. On the other hand, a secondary character doesn't have to deal with all this. He/she is there to support the main character one way or another. It can come in the form of comic relief. This is something the audience loves in a character. There's always that "clown" or that "genius" or that "dork" that people love. It doesn't necessarily mean that main characters can never be the most popular though. Just look at Goku from Dragon Ball Z.