It doesn't have a magnetosphere because it's core is both less iron-rich and because it doesn't move in relation to the surface (Which is likely caused on earth due to plate tectonics which is in turn caused by... the presence of water.)

Nobody is quite sure, but it's likely one of those chicken-and-egg issues where more iron would have caused more water to form early, which would have caused a magnetosphere which would have caused more water to stay...