It ultimately depends on the quality of the movies. The original Superman and Batman films were both very well done (Batman especially, since it had to shake of the stigma of the old TV show, and fans were wary at first of Michael Keaton playing the part).
The problem is that the DC second-string superheroes are not as well-known. Also, they don’t have the dark and angsty edge that audiences like today. (It’s been grafted on, but doesn’t really fit.) I don’t know what you can do to make an interesting film about someone like Flash, Green Lantern, or Wonder Woman: straight heroics are passe, and they are not really tormented souls.
There is some possibilities with the Vertigo heroes, though, again, quality is everything (think Constintine). I doubt Sandman could be translated well, though the first story arc is a possibility; the main problem is it’s too episodic.
Marvel floundered in the beginning because the company’s main heroes were not part of the US mythology like Batman and Robin. For instance, my wife knew the general outine of Superman, but when the cheap crook robbed the wrestling promoter and Spider-man didn’t stop him, she said, “He deserved it,” not knowing what was going to happen.
As time went on, Marvel heros became as familiar as DC ones, so the films could succeed. Also, Marvel itself was strapped for cash (and close to bankruptcy at one point) for a very long time in the 80s and 90s. It couldn’t afford to wait, and thus was willing to take Roger Corman’s money to make the first, forgotten, Fantastic Four movie. Other studios, knowing that Marvel heroes were lesser known and that the company was desparate, probably lowballed them.
DC, of course, was owned by Warner Brothers, so they could make a movie of DC characters without having to negotiate any rights. That made it easier for them.