This is a good thread...
I think it's a big part personality: being OK with deferred gratification, going away from the herd as it pertains to having the best clothes, cars, big houses, etc. Certainly some of us can do those things and still RE, but the majority are likely this way.
Luck can factor in in some cases, but to say it's a cause or determining factor is a stretch. There are plenty of people "lucky" enough to have the means to RE, but many don't because they make bad choices along the way. Conversely, there are many who don't have luck falling their way, but they make good choices and sacrifice along the way to get there.
Intelligence - I think this can factor in because your capacity to understand compound interest and other important mathematical concepts is somewhat important. Not a determining factor, however, as you can simply push the "I believe" button, or make enough money as a skilled tradesman and hire a financial professional to manage your money at an early age. You don't have to be a genius to weld, but you have to be a good welder! On the flip side, you have countless examples of extremely bright doctors who start making their big bucks in their early thirties, but because of their personality they are always buying expensive cars and trying to keep up with the herd of doctors. "White Coat Investor" is a popular blog devoted to teaching medical professionals, all likely very intelligent, how to retire early because the majority of them cannot.
So, to me, the biggest driver is personality. Personality factors into determination, work ethic, and willingness to defer gratification and steer away from the herd. That's a bigger factor than minor ones such as luck and intelligence, IMO.
