Actually, Einstein won the Nobel Prize in Physics in 1921, but it wasn't for his theory of relativity. He won it "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect."

While his theory of relativity was groundbreaking, it was still considered somewhat controversial at the time, and the Nobel committee often favored awarding prizes for discoveries that were more directly supported by experimental evidence. The photoelectric effect was exactly that - a well-supported and profoundly important discovery.
