I feel like people are jumping off the Adventure Time Bandwagon, and probably on to the Steven Universe one. While Adventure time was previously full of filler (I mean ALOT in Season 6 and early 7) they started picking up the pace with actually meaningful episodes like Don't Look and I am a Sword. Don't get me wrong, Steven Universe is NOT a bad show, (but I don't really like it myself but this isn't about me) I just feel like people just abandoned it because SU got around and SU got to the point quick unlike AT's longer, more fillerish story. I think Rebecca Sugar (who previously worked on AT) was the reason people left the show, like memorable episodes and fan favorites such as I Remember You what was what people were at AT for. When she left, what the show was to them was gone. Now, in SU, she isn't just a writer (I think thats what she was) she's the creator. Also, some things people probably wanted to see in Adventure Time she wasn't allowed to do, like certain undertones and messages, and now they see them in Steven Universe (you guys know what I mean) It's just that AT jumps around with with how fillery it is. I think that the show is coming to a close soon anyway. Come on, we may be getting a Season 9 soon (8 is confirmed). Even I have to admit that. I pretty certain though when AT ends SU will still be going (like I said I'm not crazy about it but I CAN see 10 seasons of it without getting stale)
TL;DR Come back to AT, it's getting good, just because you like SU it wasn't a reason to leave.
Also I'm making a topic about this but It can be closed if it's too specific