I seem to be dominating with Zarya. Am I special? Why don't I ever see anyone playing as her?
She's basically mandatory for most team comps, as her ult is devastating, she combos well with just about anyone, and she prevents 4000-5000 damage to her team per game (essentially the same as healing 4000-5000 per game). She puts out a shit-ton of damage if she's doing her bubbles properly, too. She's also a good counter to Genji, D.Va, Junkrat, and Winston, and goes 50-50 against Mei, Roadhog, Reaper, McCree, and Pharah. That's a lot of the meta that she just doesn't have to be scared of. The only problems she really faces are a good Tracer, Soldier, Widow, Hanzo, or a Reinhardt with a team behind him. She doesn't have much burst, so Lucio heals can be a nightmare if an enemy Lucio is dancing around behind his tanks, and she can't really charge in solo and expect to win a fight, but those limitations are pretty minor, and depend on the enemy team coordinating together (in Gold and below, they don't).
You don't see her a lot until Platinum because she requires good reactions and team awareness to do well. It's really disheartening to play as a Zarya with a Reinhardt that charges ahead solo into the meat grinder, or healers that try to 1v1 a Roadhog, or with a team that doesn't react to being flanked, so I think that turns a lot of people off. But she's very fun to play tactically. Really makes you feel like a battlefield commander, and helps you understand so much more about how to play the game.