It is definitely because of how easily he beats them. They're not even in the slightest competitive so haters label them bums when in reality the star names of the division couldn't put the beatings on these "bums" as GGG did. Example, Pacquiao destroys Algieri. Absolutely annihilates him, makes it look like Algieri shouldn't even be boxing. Everyone labels Algieri a bum, an absolute bum. Then, Algieri goes on to fight Khan and Errol Spence Jr. These guys beat Algieri, and they get credit for beating a "tough" Algieri and Pacquaio gets slandered for stepping in the ring with him. Truth of the matter is, the best fighters of the world will make good (not great) but average level fighters look like complete bums because that's how far ahead they are of the game. Mayweather did it with the avg level fighters he fought. Pacquiao did too. And GGG is doing it. Now, question is will he still be that far ahead when he steps in with the BIG name like Pac and Mayweather when they stepped in with the top fighters. If you are the best in the world, you'll make B, C level fighters look like Z level fighters. It's a lot harder to be on that level than people think. Canelo has faced A+ fighters, but has never dominated. He's even faced B level fighters and still doesn't dominate. It usually comes by a close or controversial decision. He's a great fighter, but that's why he'll never be on par with May or Pac. If he was, he would still make it look easy with the fighters that are lower tier than him and it never does come easy.
There is no fighter you can truly argue as being even a B level guy on his record. Lemiuex has never beaten a top fighter ever, Murray hasn't his beat claim is losing close, Macklin has never beaten a top fighter, nor Stevens,an so on and when presented with an opportunity to it's always an excuse for it. A great fighter can make good/decent fighters look bad but in this case he's legitimately fighting trash.
If he's fighting legit trash, I'm assuming you think Canelo would do same or better performances against the likes of Murray(KO11), Stevens(KO9), Lemieux(KO8 )?
I wouldn't pick a single one to win, and nothing shown from any of there careers says they can beat Alvarez. I mean Stevens was just used by Canelo as a sparring partner and afterwards Stevens has acknowledged Alvarez is legit. Also I never brought up Canelo but i am still down for the bet when they eventually fight.
His opposition is mediocre. But so was Larry Holmes' and Hagler's (fought three greats but all moving up to HIS division and got schooled by a shot, old Leonard with a bad eye). Those guys are hailed as legends.