Let's say you release a new Web 2.0 app as GPL and I take the code and want to make a closed-source, proprietary thing off of it. Well, I can. Since I'm not distributing the code (merely running it on my servers as Software as a Service), I don't have to contribute back. I can make additions, incorporate your additions so I don't fall behind and start making my service look better than the open source code.
BSD has come into fashion because either 1) you're not going to release the code and nether license helps there or 2) you're going to release the code in which case the GPL's length and requirements are just an annoyance. The GPL simply doesn't protect web code in the way that it protects desktop apps. And so developers feel no need to use the GPL - it doesn't offer them anything and only serves to do things like make employers nervous or the like.
The AGPL (Affero) is a different story. . .