My guess is that there are two prime movers here, albeit ones that work in different directions. The first is modern training regimens. I think it can be demonstrated pretty categorically that human performance in most every sporting venue has improved vastly in the past century. Whether it's sports medicine, diet and nutrition, equipment, training, coaching, and so on, all aspects have benefited from advances that allow athletes to ramp up their performance. I'd argue that enhanced performance leads to enhanced tactics and strategy, hence fighting arts can evolve.
Despite sundry technological revolutions, however, fewer and fewer people able to take advantage of these amazing changes have had to confront the raw brutality of a one on one fight to the death. There was a time when most folks thought nothing of grabbing a chicken from the yard, walking over to the stump, lopping the critter's head off, then watching its body run around the yard pumping blood. Do that in an auditorium these days and people would probably stampede.
In the nasty, brutish, and short days of yore traumatic amputations and grisly deaths were things folks were a lot more likely to witness. A blooded warrior would not have a problem lopping off pieces of an opponent. Outside of modern Special Forces, you'd have trouble finding an elite level athlete inured to violent death, and even elite operators are going to be doing most of their death dealing with weapons that allow them to stand off from their opponents. Bottom line is I think an elite athlete would have the capability of taking one on one fighting in new directions, I just don't think you are going to find many willing to pay the bloody dues these days.
