I think I want to change my answer. At first when I just started gnutella I really noticed a lack of swarming capabilities (and thus slower downloads) because of the lack of hosts. But since that point I have completely changed my mind as the quality and number of shared files on kazaa seems to have gone downhill. I think the RIAA might have actually managed to put the fear of God into the Kazaa people.

Anyway, now I'm inclined to say that the only reason Kazaa is so popular now is because its a bigger name between lusers, someone asks for a good filesharing program and everyone knows kazaa so they tell them that.