From what I've heard trying to sign a bigco up for anything is a lot of work, usualy far more than a small startup can afford to spend on a single customer. The company will spend so long evaluating your software that you'll have run out of cash before hearing a yes/no from them. Better to chase individuals/small biz at the start because they're easier to sell to. Of course there will always be exceptions (loopt and boost springs to mind despite not being an enterprise app).
from what i've heard about all things at the enterprise level, corporate accounts are notorioulsy sticky because of the process (it takes uncountable pages of paperwork to get anything done).
so maybe CIOs are reluctant to switch because they already buy from IBM/MS and it's just easier that way.
their loss...
This doesn't mean things are hopeless for startups, though. It just means they won't get the big cos as customers till they themselves grow larger. Or to put it more optimistically, that they will grow larger by gradually consuming the market share of the "incumbents."