I was a VC. I think it is mainly that it is a very weak signal. A good VC or a select group of high quality angels does help a company get itself in order, so raising money from a large collection of distant angels tends to be a signal that the quality of the company must be low - as they should have tried to raise from high quality individuals first.

A company with crowdfunding will have to display stronger metrics than a company with strong angels / VCs.

Imagine two companies come to pitch, both are identical in every way. One has crowdfunded a $1M seed, the other got a check from Andreessen Horowitz and the CTO of LinkedIn. Which one do you put your money in? Even the potential deal flow from those individuals helps hedge your bet.

As a side note I think that this relationship of quality and crowdfunding is true (though not always by any means). I have seen fraudulent behaviour by companies misrepresenting their data to a group who are, by a VC's standard, very unsavvy investors. This company was certainly not able to raise money from a VC, but raised two $M+ rounds on a crowd funding site. However, I have also seen good companies who don't have an 'in' to the VC world raise money in this way.