Canelo just doesn't want to fight Golovkin at middleweight, besides GGG there is no good middleweights, Canelo beats anybody at middleweight besides GGG.
There's quite a few MW's that would be more than a handful of Canelo, he probably loses to more top MW's than he beats.
All the top MW would put a serious hurting on Canelo. Especially DL. There's a reason he chose Amir Khan as his opponent.