That's a good question, I have some theories but I can't say I have an answer. I think a big part of it is that MMA kicking is heavily influenced by the Brazilian style of kickboxing which was made famous by the Chute Boxe fighters. Generally speaking they didn't really pivot when kicking, rather, they'd step, turn, and plant their lead foot out to the side to load & open the hips then whip the kicking leg through. Bas Rutten teaches this method as well. I think where the problem starts is some details got lost when everyone else started copying their kicks; they don't turn the foot out properly when stepping into the kick so they lose the power from the hips, they try to throw straight from the stance with no step and get the angle wrong, and so on. So for a while we had fighters from a TKD background like Anthony Pettis and the original Chute Boxe guys (Shogun) being the ones with the best round kicks.
I think a good round kick, especially a Thai style one is just something that's not well understood in MMA. Sylvie von Duuglas-Ittu has been fighting in Thailand for years and it was only recently that she had her "ah-ha!" moment with round kicks. She wrote a really nice article about it.
http://8limbs.us/muay-thai-thailand/golden-kick-how-to-improve-your-thai-kick Not surprisingly, both Shevchenko sisters throw their kicks that way, as do Pettis and David Teymur on the men's side.
