walmart offered songs for like $0.60, a very large advantage over iTunes, especially after they went to $1.29. However the walmart music store is now dead and no longer exists.
Amazon actually has an option to automatically add the purchased music to itunes or windows media player. Amazon also offers a pretty large library... they may not have some of the same indie bands, but they certainly had all of the same mainstream and top 40 music and still never managed to become entirely relevant.
Google is really offering nothing unique to the online music stores.
most people will stick with iTunes because it's what they know. iTunes became popular because everyone pretty much had an iPod, and it was easy and convenient to add and purchase music through iTunes. I don't see many people switching to google other than a few android users.