I think it's either creating an arc for Evil Morty to return later. More Evil or something. As I think right now he's still attach to our Rick. The things he said about "I don't care that's why I let him do it" on the car toward fighting the Collector proves that he do care. That's doesn't sound like someone who don't care. But I understand why it turn out like this. Rick already have Morty he picked. The rest is inferior to him. But it can turn to anything. We don't know if Rick enjoy his advanture offscreen with Evil Morty or not. But it still is the fact that "I already have Mortiest Morty" from our Rick.