I think it's because very few of those shows have been successful enough to sell merch. Adventure Time and Regular Show did really well and got stuff. Rick and Morty exploded and got a lot of merch. It's usually only CN/[as], Disney and Nick barely make any merch for their shows at all unless they are the heavy hitters. Even then Disney was stingy making Gravity Falls merch and we only got some. The only cartoon they really put toys out for was Phineas and Ferb. I can't tell you the last time a Nickelodeon series got toys. All they care about is SpongeBob.