Who would have predicted that Leningrad would make a comeback in the 23rd century? I think the main reason for the lack of Earth-based diversity is pretty simple: Star Trek is, as things stand, a Western show aimed at Western audiences. Studios and directors share a fear, most probably unwarranted, that moving the focus away from Europe or North America will alienate vast swathes of the public in those two continents. Of course, this is a damned stupid assumption. Setting a scene in future Tokyo or future Beijing would have no impact on the film's reception in the West, but it will boost the Box Office numbers in South East Asia.