I wonder about the companies that a VC rejects that end-up going under. How much is a firm's failure due to lack of funding vs. actual competitive forces or man

I wonder about the companies that a VC rejects that end-up going under. How much is a firm's failure due to lack of funding vs. actual competitive forces or management inability. Not saying that I'd be a shinning beacon since I would have turned down Twitter and Zynga.