I would chalk it up to intended game design. It's their game so they can choose how they want to implement each champion.
Were you around when Soraka was viable mid/top lane with the old starcall? It was one of those instances where players realized that soraka was actually a abusive mage, could heal herself permanently, could give mana and more. It was unhealthy, and not what they envisioned as a healing support. You had fed sorakas running around getting pentas by just button smashing.
If you give someone like Soraka (because I talked above) the power to go in to other lanes and spar 1v1 with champions that are intended to be in those other lanes (mages/assassins/bruisers) while still leaving her utility across the map and in team fights that wouldn't create a healthy game. Imagine a soraka being able to duel a syndra, while at the same time providing massive heals, point click silence (snare given the chance) with continued health sustain. It would create once again another abusive champion.
Riot wants there to be champions for each role, so they design champions with that in mind. On top of that they design items for those specific roles to utilize to make them excel even more at their role.