As the old man in Pet Sematary said, sometimes dead is better.
Besides, Aaron Taylor-Johnson is playing Kraven the Hunter in Sony's Spider-Man Universe. Not only could that lead to scheduling conflicts that could prevent him from playing Quicksilver again, but he probably chose the role because he gets to be the lead instead of part of an ensemble and because it's a much meatier role than the flat and underdeveloped one he played in the MCU. Furthermore, as you would probably guess from the fact that he's playing Kraven, he now a completely different physique. He's really big and bulky now and isn't lean like he was when he played Quicksilver.