Because everyone and their mother buys Vaseline first and doesn't PLAY THE FUCKING OBJECTIVE.

Honestly, play anything but the snipers. You don't have the maps or movement systems down, you will be next to worthless to your team. (Don't even try with the "but I have the best KDA of everyone on my team when I play sniper." KDA is irrelevant if the objective doesn't get done and if you aren't dying, the enemy doesn't consider you enough of a threat to use bullets on, or you aren't actively fighting.)

In regards to pub meta, you can play pretty much whatever and win if you're good enough. Skyhammer and Aura are great starting mercs to learn with, play a lot of them and use the free rotation to learn others before you buy them. My personal recommendations are below.

Fire Support
Skyhammer is solid, but I prefer playing Arty. Both are great at breaking the EV, Arty just has lower cooldown on his strikes and can use them creatively. Stoker is also worth mentioning for area denial and indoor use, along with similar weapons to Skyhammer. Kira is a lot of fun, but also very squishy and much harder to use effectively.

Assault
Nader and Fragger are always useful for the ability to kill without line of sight, and the general utility of explosives. Thunder is great for being obnoxious and gunning down slowed enemies. Rhino is a bit more situational, but very strong in his niche.

Medic
Aura is the best defensively. Phoenix is the best offensively. Sawbonez is the best for keeping yourself and 1 or 2 others in the fight, but starts struggling in keeping the entire team up. Sparks is a starfish necromancer that is probably the hardest merc to play in the game.

Engineer
Bushwhacker is great to start as you'll learn the maps, and his weapon selection makes him more viable in competitive. Fletcher is the go-to because he can assault and complete objectives, but requires more movement knowledge to make effective use of his stickies. Proxy is amazing early because new players don't have eyes or brains and will run into the same mine placements over and over. She falls off hard in competitive because experienced players can hear mines and avoid them.

Recon We've been through this. Avoid. All of them.