The problem is, calculating those things for ELO shifts ranked vastly in favor of champions that can abuse those calculations. For example, if you want Janna, Soraka and Zyra to all be viable picks that you can climb with, then cc, healing and damage must be balanced out. If you do that, you have champions like Galio, Sona, Vladimir or what have you completely outshining them. People wouldn't want to do the best for their team to win and would instead focus on dealing the most damage, healing the most damage and applying the most cc even if it doesn't net them kills, they get punished for it or otherwise prioritize it over objectives that would have a far greater influence on the game - but not on their ELO. If you account for wards, Supports could sit in base all game mashing sightstone, while if you don't, Supports have little incentive to ward at all. Even if you put in some kind of qualifier like the ward having to light up a bush, They'd just play like a 0 damage pacifist Bard warding every bush on the map without contributing.
The only actual way they can do it that makes sense is based on team win/loss.
Even if they implement what the OP suggests, how could they determine whether the game was in their hands or not? Even a 99/0/99 999 cs player could be at fault when you consider the possibility that they didn't contribute (enough) towards objectives. Consider a more realistic example. What if a team is only feeding because the 15/0/20 Anivia kept sabotaging them with their Ice Wall early in the game and they couldn't recover? What if the Anivia just got fed because of luck or something and honestly believed the Ice Walls were helping, and no one reported them because they apologized and legitimately don't deserve it. What then?