By going dark many VCs assume  that they preserve their options for getting into the game at a later point. Most VCs don't want to give you a clear "no" because

By going dark many VCs assume  that they preserve their options for getting into the game at a later point. Most VCs don't want to give you a clear "no" because they know pretty well that you might change your business model, technology or even team very quickly especially when you are in the early stages (ie pivoting). And by telling you what exactly they are not liking about your startup, VCs are afraid that you might take it personal as well (just as some people take a "no" personal). And then there are VCs who want to get some more insights into a specific market or technology to use that for one of their portfolios.