Einstein's failure to win a Nobel until 1921, and that prize's not being awarded for his work on relativity, is generally ascribed to these factors:

1. Lack of sufficient experimental proof for the theories at the time (some distrust in the early results on the precession of Mercury, and the Eddington starlight observations).
2. Failure by some members of the committee to understand relativity
3. anti-Semitism
4. skepticism generally about the utility of relativity, and whether it was physics at all.

If these factors were in fact the reason that the relativity work did not garner a prize in 1921, why didn't the relativity work merit a second Nobel prize after, say 1945? Surely the work on cyclotrons and the bomb provided firm experimental support for the validity of the theory, and by then the overall utility would have been understood as well.

(For that matter it's interesting to speculate how many Nobel Prizes Einstein should have won; but this question just asks: why only one?)