That's because US citizens seeking VC who haven't moved to Boston or San Jose usually have no good excuse not to; for VCs, it's an easy way to filter out undedicated founders. Foreigners who want to move there but can't get a visa have a much better excuse.
That assumes that VCs have a need to invest in people outside the valley or Boston. VCs would have to travel outside the country to meet potential founders (since they don't yet have a visa) There are enough founders in those two places that VCs don't have any pressing need to jump through such hoops.
