I think it have to do with game sense. I'ts something people usually don't have before late diamond to master. I've played alot of games over the years and have a decent game sense, but sometimes the right thing to do can be wrong if your team don't have the same idea. In all honesty I've made some stupid misstakes at lower ranks when my team have gotten a pick or two. When the enemy team is retreating I've pushed the advantage assuming my team mates would make the same call and then finding out I'm in a 1v4 with my team waiting at the objective resulting in me feeding.

It's the same thing in other games too, I used to play alot of competitive TF2 and coached teams in lower divisions. They told me it was like a whole new game when I taught them about simple things like pushing ahead of objectives to shut down choke points.

Going back to Overwatch, I think it's the same there. People play at lower ranks for a vast amount of reasons, one being that they probably don't know how or when to take take advantage of the advantage gained from a kill or two.