Kane seems to get a lot of retroactive hatred, kind of like X-Pac. X-Pac was never 'cool', but he was a decent spunky underdog who was a merely 6 feet tall, and his tagging with Kane made them both really sympathetic because they were a typical odd couple.
But the thing with Kane is until they unmasked him, people enjoyed him a lot whether he was heel or face (but especially when he was face because he works as a frankenstein who has had a hard life best). That continues until today where, even more than a decade after they took his mask off, and him suffering through corporate kane and everything else, when he shows a little of that initial personality he begins to get pops again. Not much, because the damage is almost irreparable, but people are eager to root for the Big Red Machine if you give them the slightest reason to do so.
The reason that split personality Kane worked for 10 minutes is because it was sympathetic. It brought back the old idea, which you either remember from before or immediately recognise because it's such a good archetype, that he's a really strong guy who doesn't know his own strength, and is driven by emotions he can't control due to people betraying him in the past because he's a 'freak' or a 'loser'. The split Kane also had the added benefit of being funny without making a total joke out of him.
Now Kane has looked like shit at the worst times, like right now. We know that's likely because he had some time off and was working out as Corporate Kane, but the time for him to be in shape is during a masked run, and he seemingly never has the right body at the right time - not since the big red machine, where even though they covered everything except his arm, all you had to see was that arm. There's also the fact that they won't book Kane to get over on anything other than a monthly basis because he exists right now to be a random feud for whoever the reigning WWE champion is, which he'll always lose, so there are no stakes since you know nothing will happen and things'll be reset.
Then you can get into how his movements and stuff have diminished, but that's got to do with his body always being in the wrong shape for the angle. Undertaker doesn't move how he used to either, but he's got all the other elements of the gimmick intact, while with Kane they've sacrificed almost all of it piece by piece, and when they do bring back the mask, he isn't the big red machine, he's 'demon kane', which nobody really wants to see. That's a shadow of the guy they remember. He looks diminished, it just reminds you of all the bad angles he's been involved in, he doesn't even wear eye make up to hide the fact it's just corporate kane wearing a bit of plastic and a bad wig.
Literally all they need to do is have Kane come out in his old gear (and you can leave the mouthpiece off, but he should grow some facial hair), and win a match, and everyone would be lovin' Kane. But they never do that. If there was a time to do it, where doing it didn't make him seem overpowered since they don't want that - he's what, 46? not his time - then it would've been during this brothers of destruction returning angle. That could've helped gotten this feud, which isn't working, over.
That was a long post to say 'kane should've come out with his old stuff on'.