Because the MM system creates artificial plateaus, that are not just based on your comp rank, but also based on your statistics, and QP MMR.
You can thank Jeff for that lovely balance system.
If you feel you should be higher ranked the only way to re-calibrate the MM system is to create a new account “Smurf account”
You essentially force the MM system to relearn your consistency level. And you have a chance to start from a clean slate where your previous stats don’t effect your rank gains as much, but will improve it drastically depending on how good you really are.
On are more serious note however: Experience in FPS games helps a lot.
Have you or do you play FPS games a lot? Doesn’t matter which.
Is this your first FPS game?
As much as I hate stereotyping in games, Bronze generally are people who have never played an FPS before or have little experience, or use the wrong control systems, or don’t have a good configuration, lack map knowledge, tactical knowledge, understand what each class is capable of, and as others have said knowing when to fight and when not to. This game isn’t a deathmatch, unless you go to custom games. It’s not as simple as running and gunning. Well unless you play S76 of course lol… then it literally is running and gunning 
But, to give you an example of previous FPS experience, If like me you played Quake trilogy games a lot, there are a few characters that specialize in Railgun tactics.
Hanzo, Widowmaker, and Mccree. There’s a cpl others but those are the ones that blindingly stand out.
As far as run’n’gun tactics:
Soldier 76, Sombra, and tracer are prime examples.
Granted each character has their own special skills but at the core they all have similar play styles based on traditional FPS games.
Some will be better with aggressive play styles while others at defensive play styles, and for someone like Soldier 76 he’s good at both. He can heal himself, pitch a tent if he wants to, or drop his ult and rape the enemy team as long as there is no shield tank of some kind to stop him.