Ludosity is a small team given the rights to a bunch of beloved IPs. They probably felt a lot of pressure to do things right when planning the roster, especially as they had neither the guarantee of DLC nor the guarantee of the game even being popular. I'm guessing the roster was selected to be popular with fans, to prioritize characters that are fun to play, and to make the most of the opportunity (i.e., if there is a character the developers really like or feel is important to include in the roster ((and Nick is willing to fill out all the paperwork to let them use the character)), I don't think they'd hold back as this could be their only chance to use that character, even if the character is more niche or a secondary character in their show).
I think that may be why we have characters like Nigel and Powdered Toast Man. If this may be their one chance to make a Nick game, then they want to give it their all!