The MP3 format became popular because it offers a good balance between audio quality and file size, which allowed people to share them easily on P2P networks back when broadband internet was more a novelty than the norm. WAV files have been around forever without DRM.
There are different degrees of popularity. The amount of people that shared files using FTP and usenet pales in comparison to the amount of users of P2P networks. I think it's fair to say that Napster and then P2P made MP3 ubiquitous.
I think Winamp, which predated Napster by a couple of years, was really the turning point that made MP3 popular. Really MP3 made P2P networks popular, rather than the other way around.
