Can someone help me figure out how to prevent my team from throwing every game?

Hiya! I just got done trying to do my promos for the 3rd time, and, even with the free win, I lost both games. The reason? We had the advantage early and mid game. We were destroying towers and getting kills all over the map. But then we lost one fight. A single fight. And from there the enemy was able to take down a turret, and inhib and a nexus turret. My big question is: Why is the game decided by a single team fight at 30 minutes in? A team with the advantage shouldn't lose because of a single lost fight!