Considering how the fight ended with the low blow controversy, and considering the fight was competitive although Duran was definitely ahead in the fight. Why was there never a real demand for a rematch ? And why did it not take place ?
I don't think ken had the backing of a big promoter plus he lost to Guts in Japan. Also I doubt Duran would push for it as ken have given him a hard fight.
The Guts fight was 3 years later, so not a great excuse for no rematch especially given the controversy of their fight at MSG. I think Ken was offered a rematch in Panama but the venue (and purse) were relatively poor, so he turned it down.
Duran lost a decision in a non-title fight to De Jesus just five months after beating Buchanan, which I think caused a major shift in his focus. After that, avenging and ultimately mastering De Jesus became his main obsession, especially given his intense dislike of Puerto Rican fighters, as per Christian Giudice's biography. Buchanan was a less pressing matter than De Jesus by the end of 1972. Also, Buchanan really started losing the battle of the bottle not long after and was falling out with all his managers and trainers by the mid-seventies. He didn't give of his best against Ishimatsu - had he won that, he almost certainly gets the Duran rematch with the undispited title on the line. But it wasn't meant to be, and he pretty much drifted away from the sport on a serious level after that.
Think I remember that Duran's management and MSG boxing had a big falling out and that caused the fight to not be made.