In my experience, it mainly has to do with macroplay. That is... knowing which objectives to take and when. A little also has to do with how well you know the mechanics of a champion but I've seen people in Bronze who are amazing mechanically with a champion but have tunnel vision and only go for kills cuz they want the sweet KDA over that chance to take tower or dragon. Similar problem in silver, though I've seen more bad calls than tunnel vision where a mistimed baron call leads to being aced and then a loss as the other team pushes to victory. Gold is 50/50. You'll either have people who know the calls or people who don't and the team that does, wins. Plat is like a box of chocolates... you have boosted players, players who think they know best but don't, and people who actually know... kind of a dice throw, especially plat 5. In diamond, people generally know what they're doing and when to do it, but the margin for mistakes is very slim and even if you just make one tiny mistake, sometimes it can cost you the game.
This was my experience from climbing, it might not be everyone's view on it, but it's at least one view.
Hope that helps!