For instance, Big Hero 6: The Series brings back all the movie cast except Damon Wayans, Jr. and T.J. Miller, while Dreamworks Dragons brought back Jay Baruchel, America Ferrera, Christopher Mintz-Plasse, and (oddly) T.J. Miller from How to Train Your Dragon but recast everyone else
I thought it might be because of Miller's multiple assault allegations that led to him being recast in How to Train Your Dragon III, but I checked and the timing doesn't line up, unless the Big Hero 6 producers can see several months to a year into the future.