You know people look at a lot of guys in the industry and say better heel, you'd get it with ric flair, and you get it now with guys like orton and john morrison. Very rarely do you hear people say he was a better face. But rest assured it is sometimes the case. Hulk Hogan was so good as the face of the 80s because he could read a crowd and as a heel in the NWO, he was good but because he could no longer get the reaction of the crowd from playin to them because he was heel so it was more the fact that he was at the centre of the biggest heel stable since the four horsemen that were immensely powerful that gave him his heat.
To bring up another random example, and your going to
but it's true. 1-2-3 kid, sean waltman, never really over but over enough for people to back him and see him as a future player. He has now a unique bismirchment/honour in the process known as x-pac heat. He was one notable guy to recieve this and what it means is he was booed not because he was heel but because people genuinely didn't like the guy. And why.....because he wasn't as good a heel as a face, he had the babyface looks and athletic skill that could make him and underdog type figure and he was being used as an enforcer for a powerful group of bad guys. He came across as the stupid mafia guy in films who always gets killed first because they don't really know what they're doing.
Undertaker is this too. You can see similarities between the face turn of the undertaker in '92 with the orton's face turn this year, just as the feud between orton and legacy could have turned either side, so too could the jake roberts and undertaker angle. What happened is history and what it means is that people already had a reservation that ie viper orton was more of a face or that undertaker was a more of a face. You can view these guys as morbid superheroes, likeable in a different way. They fight for good whilst suffering their own fissures in character, and that's puts them over, the sense of moral justice, they are like cartoon good guys but darker like watchmen.
While randy orton was a snake, you always got the sense he could if he had to take care of things on his own, but he was just clever and got help, while legacy were parasites living off the turbulence of orton. In truth it is a tribute to characters with attitude, the reason orton was the one that got over was becuase he was one bad dude who didn't take crap from anyone, not even his boss and people ended up seeing that through his heel persona and liking him for it. It is just the same with taker, he wasn't a mortal guy, he didn't ever lay down for the opposition, he always got up, and people see that and admire it.
To answer your question, taker is just quite simply a better face, or rather his character is a better face and it is difficult to turn him into a bad guy unless you start having him cheat to win and doing other heel attributable acts. During the ministry, undertaker was never really top heel, not really. He was always just behind HHH, or austin or the rock or any of the top dogs, which is whe he returned he became instantly face again, its his character it is just face, he can just do face more easily. His biker heel turn was similar, he was on the bad side but you didn't get the feeling he was a low-down type of guy, just someone fighting for his own agenda, undertaker always had trouble being really over heel because his charactr and his ability wouldn't really allow it.
Some people face are good to watch, they make you mark out or come back to see the good/evil struggle and that is undertaker's place in WWE history, we shouldn't want it any other way, he's played his parts perfectly.