It's all about the money, phleq. Big fan bases mean big money. HUGE fan bases mean HUGE money. As you said, the BSB following is massive (biggest domestic series in the world if I'm not mistaken) Britain hosted the first SBK race, is the homeland of (statistically) the greatest WSBK rider ever, and the only nation to hold more titles is America. Now, Britain hosts three WSBK rounds. It's the spiritual home of the World Superbike Championship, and has the fan base to boot. That's why you've got a third round. Now think of Spain, and to a lesser extent, Italy, as the same "Spitirtual homes" for the MotoGP circuit. Perhaps not as firmly rooted in the historical department, but the fan base for the 125 and 250 GPs is massive, at a time I belive it outstripped that of the 500s, and 125s and 250s have become the real feeder classes for MotoGP. So the cream of the 250s more often than not ends up on the MotoGP bikes, and thier fans will follow them. GP racing and SBK racing are different spectacles, and while you Brits have proved how much you love the Supers, the Spainiards and Italians have proved how much they love the GPs.
There are a lot of countries who suffer from not having good representation in the GPs, so don't think Britain is alone. Considering our size, Australia does pretty well, but I belive there's a lack of good Germans in the series, for example. Germany has the biggest bike market in Europe, and the crowds at the Sausage-ring are huge. In my opinion, it should be a "World Championship", with only one round per country, and a more divers grid, but the demographics speak for themselves.
By the way, you should keep an eye on the smaller classes. I think little Smith will have a few more compatriots down there this year