Hmmm. Comedy is sometimes overstuffed with contemporary references which lose their appeal and meaning with time, but this seems like a bit too sweeping of a generalization here. I actually like Shakespeare's comedies, for the most part, and find plenty to laugh at in the comedies of Aristophanes, which are obviously far more dated yet. Conversely, it certainly isn't the case that all drama ages well. I wonder if it's more that comedy consistently gets less respect; perhaps old comedy is not as well regarded as old drama for the same reason new comedy is generally regarded as less substantial than new drama.