They do give more than one on very rare occasions. Bardeen is the only one to win two physics nobel prizes, marie curie won one for chem and one for phys and pauling won one for chem and one for peace. In general the nobel prizes are filled with controversy and on average nobel prizes are awarded somewhere around 50 years after the initial discovery (and nobel prizes are not rewarded posthumously). So it could be that einstein just died to soon. However, personally, I believe that the importance of Einstein's relativity is over stated by the public at large. Bardeen, for example, who was the only guy to win 2 physics nobel prizes won the first one for inventing the transistor (transistors, btw, are the core of all electronics and computers and the reason for the possibility of the digital age) and for the development of BCS theory (he's the B) which is a theoretical model of superconductors and superconductors, if we can ever create room temperature ones, will likely change the face of all technology again. For this Bardeen got two nobel prizes. Einstein's relativity (which was by no means his only contribution and personally I think his work in stat mech is of more importance) will likely never have a profound effect on the average person. Therefore, although he was certainly an obscenely brilliant man and his work has changed the way we look at light and time forever if he would have gotten 2 I think I might have called foul. But that's just my humble opinion.