There's no excuse why they didn't put any real effort into pushing him after the Mizdow thing. Before that? Eh... I didn't agree with it, but I could understand that Sandow (while he got more heat than he ever should have given his status, or people give him credit for), wasn't getting this massive amount of heat and hate that warranted making him a top heel to use and help get over a babyface.
However with Mizdow he got MASSIVELY over. For 6 months he was getting chants every single show, getting pops for his part of the Miz entrance and everything he did in matches no matter who it was that he was facing (even Bryan and Reigns in a tag), or in any situation (he was the first guy and one of three to get a big pop and positive reaction at the Rumble after Bryan was eliminated). No one besides Daniel Bryan himself, in the last 5 years at least, has been that over with the live audience. For whatever reason, he made people laugh, but he didn't just do that, he connected with them. WWE threw all that in the toilet, people wanted to see Sandow as himself. He still got a big reaction when he cut that one promo as himself recapping the imitation gimmick, and then the next week WWE throws him into a Macho Man costume. That's what really gets me... if they had tried seriously pushing him and he ended up with little to no reaction by the time it was all said and done, I could've accepted that. But WWE didn't even try and now even if they try to bring him back and it doesn't work, nothing will excuse that WWE flushed away a big opportunity with Sandow. And what's worse is if they could do that with Sandow and considering how over he was, they could do it with just about anyone. You need to be Bryan-peak level over to even have a chance at getting to the top... and even then it's not a sure thing unless all the pieces fall into place (like with Punk leaving and the fans utterly rejecting Batista as the Rumble winner).