They don't seem being getting enough $$$ for these prequels and people are tuning out. And the original cast are not immortal for Fox to disregard them for another movie when we know the movies they appeared in were more successful.
They could easily tell these stories in the present just like Deadpool and the rest of the MCU. Prequels after prequels is overkill now.
I wouldn't exactly call these films "prequels". They would have to set up key points that leads directly to X1 to be considered a prequel and First Class is the only film to fit that description since it has the same opening scene as X1. Days of Future Past is more of an interquel with a reboot ending that erased the entire original trilogy so these new films aren't leading up to anything. Both Apocalypse and the next X-men trilogy would be considered anthology films like Rogue One is to the Star Wars series.