The main reason is the notion of preferred stock. Corporations have it, LLC's and S-Corps don't. VC's want preferred stock so they can:
LLC's also pass through losses every year (like a sole-proprietor) while the S-Corp has limitations on who can own them (e.g. corporations can't own S-Corps nor can foreigners).
You might want to compare the various structures available here: http://www.themoneyalert.com/Corp-Entity-Table.html. I would suspect that this difference is the one they aren't comfortable with:
Members can set up structure as theyS corp/C corp:
Shareholders elect directors whoIn short, they have templates for S & C-corps but LLCs require extensive definition ("operating agreements") because they are less implicitly structured.
manage business activities.
As Jarie said, the main reason that LLCs are not preferred vehicles for raising capital is that many investors want "preferred stock," which will give them a leg up when the company goes public and all the private stock is converted to public common stock.
Basically, if a company wants to raise VC funding at any future stage, LLC is not the ideal entity type. Normally, startup businesses want to pass through losses in the first years of the business, so they choose an LLC or S Corporation. Then, when they want to get financing, they change to a C Corporation to issue preferred stock to investors.
While it is relatively simple to convert from an S Corporation to a C Corporation tax free, switching from an LLC to a C Corporation can trigger difficult tax issues that require an accountant if the business has bought and sold many assets. This makes LLCs a less optimal vehicle for passing through losses during the startup phase.