Wages at the higher levels of education are not significantly lower in Berlin than anywhere else in Germany. Berlin (and Brandenburg) still has a somewhat higher levels of unemployment (at 7.x and 5.x%, respectively), but that doesn't really do much for such a project. Land is relatively cheap, but I doubt that makes much of a difference.

If I had to speculate, Tesla is concerned about finding people willing to relocate, and Berlin is a rather attractive destination even for people not speaking German.