Sorry it is not the best selling, and wont ever be.
Fallout sells are as big or better than oblivion.
Last year in an interview inquired about TES V, Todd stated more or less:
"Yes, we dont discard a sequel for TES series, big sucess with public, but now we have other priorities". Note the "don t discard", and the "other priorities".
Refresh me, what was the time between morrowind and Oblivion ? And what is the time between Fallout 3 and... Vegas ?
Sorry to burst your bubble but THE prime product now is fallout. TES is second. The company is simply seeing more potential in the Fallout franchise than the TES franchise. Hence
Silly put: BANG BANG attract more people than CLANG CLANG. And business is business.
Hence my words:
Bethesda will develop TES V when they tire or people tire of fallout. So don t expect anything before 2012.
We will see TES 5, but nowhere soon. And i really hope its better than Oblivion. Combat aside, Oblivion was a step back in design.