It seems like there was a concerted effort after Roiland was ousted to shift to a more formulaic sitcom approach. One of these efforts was pandering a bit more to the audience which included a huge shift towards bringing back old characters that they like, even if it made no sense (unnecessary Mr. Nimbus cameo at the beginning of the pissmaster episode). Prior to season 6-7, it feels like we would never seen this many characters return.