Earlier quoted context omitted.
>Einstein supposedly won for "especially his work on the photoelectric effect" rather than relativity. just adding to this, this is because relativity wasn't experimentally verified (i.e. not sure if it's reality) at the time.
Also, the prize is about the greatest benefit to humankind according to Alfred Nobel, not the most impressive research. Arguably, the photoelectric effect fits that notion better than GR or any other of Einstein's research. Besides that, Einstein received the prize in 1921, whereas the Eddington experiment in 1919 generally counts as the first experimental verification of GR.
Today we could argue about it due to the importance of solar panels, but that was hard to forecast in 1921. Also, without GR there would be no GPS so it's not like it doesn't bring benefits to humanity.