Like in the question. Why neodymium magnets (Nd2Fe14B) are called "neodymium magnets"? Why not boron magnets? Or iron magnets?
"iron magnets" are the regular kind of magnet.
It's a rare-earth magnet, and the key ingredient in these is the rare earth metal. In this case, the Neodymium. One probably could replace the boron with something else and change the ratios a bit (probably getting a weaker but still effective magnet). The rare earth is what makes it so strong, so it is named after the rare earth metal it contains.