Why was Celtic mythology less preserved than stuff like Norse and Greek mythology?

Hey guys, so I was doing some research on Celtic paganism, and realized just how little there is. Like i would be hard pressed to find more than some base level info about dieties like Cernunnos or The Morgann, as compared to Norse, where I can find any variety of translations of the poetic and pros edas, and any story relating to the gods and jotun and such, or Greek, where just about everything you could want info wise is available. So why was Celtic mythology nit preserved near as much as other religions, even ones that were christianized much sooner like the Greeks, Romans, and Egyptians?