1st: The meta doesn't favor adc mains at all.just like WoOxer said if High elo players and pro's drop that play adc it's expected for lower elo.If all you know is adc, then that completely changes it would have an effect.
2nd: At lower elo's its not so easy to climb.Not only do you have to be better than the others.You also have to hope your team is also better. At higher elo's people know what they are doing,they know when to stop inting and doing stupid shit. In low elo its not "get better to rank up" its "get better, and hope your teammates are also getting better" otherwise 1 person knowing what they are doing will not be able to climb.
It's like running a relay race, you might be usain bolt but if your teammates don't know how to run, it'll never matter and you will lose.Meanwhile the enemy team has the slowest runners ever but ALL of them know how to run so they will eventually win,because your team is stuck.Everyone has to pull their weight.
in low elo:Player either don't know how to pull their weight or give up.If they fall behind it's over.they feed without a limit and keep trying to play like they are ahead, which won't work out well.
In High elo:Players might fall behind but they know how to get back and pull their weight.
Ofc there are people in almost every elo who never stop feeding and don't learn how to play when behind.But I can assure you that in Low elo you run into these players more often than you do in High Elo.