I understand the kick you describe, I am TKD too.
However for many they are not coming from backgrounds like TKD and for them to use these types of kicks they would telegraphic too easily for another fighter to move out of the way and counter it. Heck I'm sure like me you train to counter this kick as well ... shoot backwards but preferably to the side and counter with a kick or punch or combination of techniques. A missed kick like this would leave the person open to much worse under MMA rules.
The spinning back kick is awesome especially in the hands of somebody that understands it like GSP or David Loiseau or Ivan Salaverry, all with TKD or Karate backgrounds. However another kick that can leave you exposed and why it is used sparingly.
I think that you will find is that many of the people coming into MMA are not coming from traditional backgrounds such as TKD and the reason why kicks like this are not used much. I however was pleasantly surprised 3 weeks ago when my own Instructor competed for the MFC. At least 2 others on the card were from TKD backgrounds with one coming from a Militich school in Fort McMurray, AB. Though some kicks were used, it is more punching and clinching and takedowns that occurred as that seems to be the basis of this sport. Lots of knees and some elbows, a few high kicks, some low kicks but again mostly punches.
Just going from my Instructor's frame of mind, or what I believe, you find your distance with your hands. Knowing your distance kicks may present themself and that is when you take them but your are more likely to use your hands especially when no resistance presents itself. And for him his opponent knowing he was from a TKD background fully expected him to kick and his idea was to shoot in and catch the kick and so had his hands not in front of his face to protect himself but out to the sides. My Instructor jumped on this opportunity throwing the jab and then the power hand dropping the guy to the ground where in desperation he grabbed for the ankle thus having my Instructor to sprawl with his free leg. Then after deliver several hammer fists open a cut, his opponent let go of the ankle to protect his face only then to have my Instructor move to the side, use the friction grip, roll on to his back while pulling his opponent's back to his chest and then choking him out with the RNC ... all in 45 seconds.
The thing is, you do what presents itself. Fighters knowing you come from a certain background may be scouted a head of time and the fighter will change their fight to how the other fights. Perhaps if my Instructor was striker his hands would have been protecting his face leaving his legs open for kicks. If my Instructor was just a grappler/wrestler then his opponent would have been ready for the clinch up or shoot.
What I do see in MMA is yes, kicks are not used that much. The hands move faster and provide less chance of being unbalanced. I did however see a few guys that did do high kicks land on their butts which can't be good. It comes down to those that can kick will kick when the time is right ... much like GSP does. Those who punch will strike more. Those who grapple/wrestle will do that more. It all stems from their background and those that don't train kicking or at one time did not train full time kicking probably are less likely to do it.
And lets be honest, TKD is not a top sport the world looks to for MMA and probably the opposite. Most MMA guys and the world for that matter look down on TKD though it has many valuable techniques. If I had to start over and was purely going to do MMA I would probably stick with BJJ and boxing first and foremost. I'd sooner or later pick up a kicking art but I'm not sure at that time it would have been TKD, however knowing what TKD is now I certainly would go to the source of the best kicks around ... and that is TKD.