Long jumpers jump from the same leg every time - in Markus Rehm's case, his take-off leg just happens to be the prosthetic one.
Plus (and this is the biggie with regards to all prosthetic appendages IHMO) - he has one leg which effectively cannot get injured. He's less likely to suffer any training setbacks during his build-up to major events. If his prosthetic leg snaps a week before the worlds, he can simply replace it. With a real leg, there'd be no chance for it to heal in time. Hell, he doesn't even have to deal with blisters on one of his feet.