1- Inelasticity of demand : Just look at windows, Word,...etc many companies "waste" hundred of millions of $ in software that's loathed by their employees, yet they still force feed it in the name of standardization. The users don't get to vote on the products they use unlike their consumer mkt counterparts (with their wallets or attention.) so change is very very slow.
2- procurement process lack of transparency : Yup, it's still an old boys club out there & the company that gets a signature on the dotted line is not always the one with the best products. Relationships & brand name matter enormously, esp if you're a clueless CIO who's climbed to the top mostly on your corporate politicking talents.
I stil think that the best way to approach the enterprise market is to have an acqhire exit in mind. grow big enough to attract the attention of IBM, Oracle & Microsoft. (Yammer stands as a recent example, but IBM made dozens of acquisitions in the 100~500 Mil $ range.)