We thought Season 5 would be their last season but do you notice Cartoon Network really didn't say Season 5 would be the final season?
Okay, When they renew Adventure Time for Ninth Season. They say Ninth Season would be their final season straight away!
It doesn't help the fact that Deedee Magno-Hall recently post a picture of her and the cast recording their voice and the production codes on the screen states 609 and 610 which is likely to be Season 6 episode 9 and 10
Because it was decided with 100% certainty that the ninth season of Adventure Time would be its last. As for Steven Universe, it is, or was, unsure if the fifth season will be the final one or not. That's all there is to it.