That would be difficult. Sentai and Kamen Rider tend to be used as a vehicle for young idols to raise their profile so they can get more "grown up" roles in dramas and films. It would be hard to keep a cast on for more than a year - they would want (or their agents would want them) to move on to new things, and would probably be worried about typecasting as well. If a new cast were introduced, that would put the show at risk of "they changed it, now it sucks" and the show losing its popularity
Hideaki Tsukada said in an interview that he'd tried several times to interest Toei in a second series of Dekaranger, and they'd turned him down. Dekaranger was a big hit, so there must have been a pretty strong reason for them to say no
