Alyssa Rosenberg asks this question in a column she wrote and makes some interesting observations:
- Louis CK easily filled an hour of comedy with his show.
- Law & Order, though an hour long, has always been essentially two half-hour dramas (The cop show, then the lawyer show).
For the argument that comedies have less storytelling to deal with than dramas, I could find plenty of counter arguments. Many of your standard-fair procedural crime dramas are quite cliche and follow essentially the same handful of story lines over and over. Conversely, there as some very dense and layered comedies that take an entire series to get to some of the punch-lines (such as Arrested Development).
So, for every argument why one is an hour and the other is a half hour in terms of complexity or story, it seems we can find an example to counter it.
I don't know the answer, but one theory I'd put forth is how the pacing of a comedy tends to differ from that of a drama. Simply put, a comedy tends to need a much faster pace. It's supposed to be (or historically has been) a light show meant to make us laugh and forget (or laugh at) reality. As such, it needs to have a fairly quick pace full of jokes and one liners. And that's hard to do for an entire hour.
Conversely, a drama can actually benefit from slow parts. The overall pace of a drama tends to be slower and varied. Giving more 'elbow room' of an hour simply allows the drama to pace itself better.
I'd argue that's why 'funny' hour-long shows (such as Better Call Saul) aren't strictly comedies, but 'dramedies'. They can be plenty funny, but since they mix in the pacing of a drama, they can much more easily handle the full hour pacing.