I also think it is the conditioning. The athletes that we see now use explosive speed allied to high levels of agility in turning and sideways movement which is hard enough. Develop these athletes to also carry minimal fat levels with muscle strength and conditioning creates a leanness and athletic speed that the joints and ligaments just cannot deal with. This is my opinion only, but there must be something in it.
You can improve the strength and resistance of a muscle but you can do very little with ligaments and cartileges. They're gristle and elastic, there will be a snapping point.
It used to be hamstrings, calf muscles and achilles tendons. Explosive speed. Strength and conditoning coaches have helped to reduce these muscle injuries to now be replaced with joint and ligament injuries.
Of course the pitches (half grass half weave, watered like a skating rink) the carpet slipper boots that offer little if any support, and the overall speed and intensity of the game are all contributing factors. But the footballing athletes we see nowadays are completely different from those of the 80's and 90's. In fact they are totally different from the make up of players from 15 years ago.
I wonder if rugby has similar issues, because again the modern day rugby athlete is completly different. These used to be huge, hairy ar$ed brutes gouging lumps out of each other with a couple of flying wingers. Now they are all built like brisk sh*thouses still but they move like 100m runners. I wonder if the types of injuries they suffer have evolved as well.