Earlier quoted context omitted.
Although I don't agree with Randall Munroe being in Nobel Prize for Literature league, even if he was he would win it in his sixties.
To finish off why this is the case.... Because you need to wait for the generation who enjoyed his work during collage to be the political / social elite.
That looks like overly simplistic explanation. Literature Nobel Prize are usually given when someone's body of work stands the test of time although Lit. Nobel Prize has more than it's share of controversies.