i think its a question of mentality... in the us, they made the ufc rules on the ones already made in boxing, considered a "clean", sportsmanship sport. If we follow that idea, soccer kicks and stomps is considered dirty, in the sense that its so brutal and it looks like hitting a downed opponent is not showing sportsmanship.. but in time, if pride keeps their ruling of allowing those, mentality will eventually change.
I havent looked up, but i dont think any1 has even been severely injured by stomps or soccer kick... the most brutal one i remember was wanderlei silva stomping kondo with his hands holding the ropes for more control while stomping, and kondo did not suffer permanent dmg..
On the other hand, if u look at slams, legal in both org, im sure every1 has seen randleman slamming fedor.. first, im not even sure fedor was really ok after the fight.. second, even if he was, its only cuz fedor... is fedor

. Any1 taking that slam would of been seriously injured and without exageration, he could of died of that slam(watch his neck getting twisted).
So i dont think stomps and soccer kick are illegal because of its effect on the fighters, but its just a question of image for the sport(and in US, its already hard to have mma fights in any state). In the future, i think the universal rule will have soccer kicks and stomps, 5 mins rounds seems to be the good format, elbows probably will be allowed, and i hope the fights will be in a ring, but with nets below the bottom ropes(like in vale tudo) so u dont need to restart the fighters every 2 mins.