Look up Smogon OU Tier list.
Whatever Pokemon you see besides gen 7, megas and legends except Mew and include the likes of Gengar, Talonflame, Gliscor, and Gyarados...that is your answer.
I'm not an advocate of "use the best mons possible" as I like a little variety and different playstyles, but sadly that isn't how the majority of Pvpers play. It's like gen 5 OU with people spamming weather, Heatran and Gliscor cores. Call it a complaint or me being salty all you want, but that is the reality we usually face.
It's kinda silly to explain why the best mons are being played the most, because that's what they are. The best.