Technically, Einstein won the Nobel prize for "his services to theoretical physics and especially the photoelectric effect", but in actuality he won it for his work on relativity.
The reason the Nobel committee did not just give him the award for relativity was because until 1919, there was no way to experimentally verify relativity theory, and then in 1919 when it was experimentally verified, there was a significant increase in anti-semitism following the end of the first World War and a great deal of hesitation to award it to Einstein for what was considered an incredibly complex theory that few people could understand.
Because of that, a compromise was reached, and Einstein would receive a deferred award for the photoelectric effect, which was much less contentious. But everyone understood that Einstein had won the award for his work on relativity.
He does not because it is not true. Einstein won his Nobel for his explanation of the photoelectric effect, which experimentally confirmed quantum theory. Quantum theory has been waaaaay more impactful than relativity, then and now.
The reason the Nobel committee did not just give him the award for relativity was because until 1919, there was no way to experimentally verify relativity theory, and then in 1919 when it was experimentally verified, there was a significant increase in anti-semitism following the end of the first World War and a great deal of hesitation to award it to Einstein for what was considered an incredibly complex theory that few people could understand.
Because of that, a compromise was reached, and Einstein would receive a deferred award for the photoelectric effect, which was much less contentious. But everyone understood that Einstein had won the award for his work on relativity.