Why don't they put out full seasons of cartoons?

I was in the neighborhood Target buying underwear, fancy salsa, and other $#!%.... And I saw a Gravity Falls DVD. I picked it up thinking it was season 1 box set, but it was only the first six episodes. I see they do this with many animated TV series. Why? Why can't sit around in my new underwear, eating fancy salsa, and watch the whole first season of Gravity Falls?

Maybe they don't think the show is popular enough to warrant a full season box-set (I honestly don't know though). I've seen animated shows get full season sets before.

mo money mo money