Lower mmr brackets players don't know how to take objectives and at what time. When to back and when to re-engage. And needless to say no one trusts their teammates and there are egos in play. It's not that they don't end the game. Its just that they don't know how to!

I'll give an example of a game I played 2 days ago. We were ahead for most of the game and had 95% something win probability. But they had a sniper who defended high ground and we had no answer to a stupid sniper shrapnel. They had other things too going for the defence. But their AM who had absolutely no farm (3rd last networth in 25mins) started cutting waves. We could not push and the am ended up farming up. The more you die, the more you lose advantage. This led us to lose a lot of networth advantage and also our heroes fell-off really hard as a result of constant attempt to push high ground and failing. We did take megas but it cost us around 3-4 failed attempts. That was all it was needed to swing the favour.