Well, only a shoddy craftsman blames his tools, but if you work in a design or ad agency everyone uses the Adobe monoculture because no one wants to deal with potential asset pipeline hickups when under tight deadlines. I'm sure there's freelancers who prefer gimp, but in the industry it's far from accepted, most designers will only know it exists but won't bother trying because they've been cosy with PS for over a decade.