Exactly. This was not a prime time television show. A cartoon aimed at kids has to be easy to follow. Look at Clone Wars. They have to make painful cuts to contain the story. Arcs rarely go longer than a few episodes. And continuity is spotty at best. Clone Wars barely has it. Batman barely had it. Save for the villain origin episodes, many of Batman's episodes could be completely out of order for all we know. X-Men however, did have a clear continuity for at least the first 3 seasons. It focused on characters with big personality and cool powers. I think that's why Kitty likely never appeared. How much combat could she have gotten on that budget with her phasing power? Likely the same reason Nightcrawler was relegated to cameos. Wolverine and Rogue were the brawn. Beast was the brains. Storm, Cyclops, Gambit and Jean had the big power. Jubilee was similar to Gambit but at least she could be shown fighting. I'd also guess the team couldn't be much larger hence why Angel and Iceman were not in it. Like Justice League which only had 7 at first and only used a portion of the cast for most episodes. And JLU which had many characters but only used several characters on a constant basis while the rest only appeared in one or two episodes. These were really the two first ensemble superhero shows. I think as they've gotten better, they can fit more in like X-Men Evolution and Wolverine and the X-Men started to do.