In B2C, if you have 10,000 users, VCs will just start to look at you. In B2B, except in some specific instances, if you have 10,000 users you're already a million dollar company.
Thus, in B2C the risk/reward inflection point occurs much later chronologically than in B2B, and for a B2B business, I, personally, think that there is far too much ambiguity before that inflection point.
Remember, the standard startup model is, "Fail."
Anything you can do to not follow that is good. ;-)
If you consider that equal to the first traction burst in B2C, then I'd argue that the ambiguity for B2C is just as large, if not larger, in the early days.
It is my no means an overall risk / reward inflection point (and if you are referring to 'this company is guaranteed to exit' I agree), but as an early 'this might have something' B2B can validate fast.
Is there even such a thing? Anyway, it's not that VCs don't fund B2B startups... they absolutely do, and there are even firms that specialize in that space (or at least have a heavy focus on it). A16Z, for example, are known for investing heavily into enterprise plays:
The current mold of web/social/mobile/location startups is the exception to the norm in the history of high tech startups.