My high school science teacher told us that he was given the prize for the photoelectric effect because that was a well understood mechanism by the time of the awarding. It was generally understood that relativity was going to be a bigger deal (concept wise), but since it was still so hotly debated / not yet thoroughly accepted, the commission wanted to ensure that he was given a prize before death (the award is not given post-humorously for some reason) and given for something that wouldn't be later rejected.
"In 1905 Albert Einstein published a paper that explained experimental data from the photoelectric effect as being the result of light energy being carried in discrete quantized packets. This discovery led to the quantum revolution."
