The turbo and mmpr powers were destroyed. Zeo powers probably hidden in cave of deception. They were rarely used after zeo season. Aside from Justin, no one had access to ranger powers. And astronema's army only attacked Angel Grove. They never said they were attacking other places on Earth. So the former rangers were powerless and scattered unaware what was going on. They probably only learned about it later. Returning cameos wouldn't be as entertaining. They repeated it in mega force with an army of footsoldiers and every ranger came back. It made the current team look weak and felt pointless for being so short.