Bethesda has much more resources than Larian, a much higher advertising budget and name recognition, a much higher profile, etc. I've seen TV advertisements for Fallout (even with a DVR to skip 99% of commercials), and mentions in general computer publications, but with D2:ED I have not seen any advertisements at all, and references are restricted to gaming or RPG websites, as far as I've noticed.
A quick Google search brought up an article stating that in the first week it was released, Fallout 3 sold 4.7 million copies (PC, PS3 and Xbox) in the US and UK, giving a profit of $300 million for the week. Another article said total Xbox sales were 1.14 million in the US. The Xbox version of D2:ED apparently sold about 40 thousand copies in the US for the first week, and 115 thousand in the first 5 months.
Lar mentioned there were several reasons they couldn't get the Xbox version of just FoV published. If you want to compare Larian to Bethesda, though, the relative projected sales numbers would have to be part of the difference. There could also have been publishers that would have wanted to release FoV, but didn't have the resources right now to handle another game, etc. Maybe some were just not interested in a single add-on, but might have been if there was going to be a series of them...
