Nowadays it's all about merchandising and brand identity. Add the name "Los Angeles" to any sports franchise you're immediately going to increase its marketability and value.
I may have my facts mixed up, but I did read somewhere that the "of Anaheim" tag came about when the Angels entered their stadium agreement with the city. One of the stipulations was that the team kept Anaheim as part of their name. So they switched to LA and tagged Anaheim on the end, those crafty devils. I mean Angels.