I think its a mixture of factors which stacks on each other.
Stage type vs Crowd type/location/occasion
High/big stages generally suggest perforamces on it would be solid, together with a larger than average crowd who would confirm be infront watching. Big stage for padestrains, or for people who don't pay or don't appreciate the music played on stage don't match. intimidating is a farely accurate word here...
Band-audience interaction
If bands insist on starting their set without at least trying to invite people forward/communicate with audience when nobody is infront of the stage, then well there goes it. Unless its hell of a crazy rock solid performance and heng heng got a group of friends who decide to go forward in the middle of the set, it would most prob remain empty for the rest of the set. Sorta like frontman's responsibility to attemp to hype up the crowd a lil before performance. Like a catalyst, a small group of people would induce more ppl to go forward, being the kay-pee-oos we are...