While Boyle isn't the most consistently bankable director out there, Fincher is hardly a guarantee for box office success, either. A lot of his successful films are more attributable to the stars he attracts, and even that's not a guarantee, given the somewhat underwhelming runs that Fight Club, Zodiac and The Girl with the Dragon Tattoo had overall. I imagine that studios would prefer working with Boyle over Fincher as he purposefully seeks out lower budgets and isn't anywhere close to being a complete control freak, even if both are still pretty picky about what projects they ultimately pursue.