Know your market
the most common pushback we got from venture capitalists was [...] who's your market going to be?' I think the passage of time has vindicated [our] vision."
From that bare quote from the V/Cs, I'd say it's the V/Cs doing their due diligence. If the V/Cs think there's no market for your product, you have to show them that there is. V/Cs generally aren't in the business of just giving money away with no prospect of return.