Yeah, it's also due to him playing the role so well, both in and out of the ring.
Honorable mention goes to the "Stone Cold" gimmick, which not only saved Austin's career, and not only defined a the wrestling era of the late 90's, but, it saved the WWF from certain bankruptcy. And, like in the Undertaker's case, the man lived the gimmick, which helped.
i say taker, mainly because on paper his gimmick sounds silly and much outdated, but it really rolled with the times and is still loved by mostly everyone.stone cold was amazing, but i feel like its not really a gimmick, its mostly him being himself. unless u say that being himself is his gimmick
I can understand why people are saying Undertaker, but if anybody else played Undertaker besides Mark Calaway, it would suck. The actual gimmick is bad, or, at least, the unaltered, early intention of the gimmick is bad. It could've quite easily gone the way of the Boogeyman or Papa Shango, but Calaway brought a charisma and a presence to the character that made it intimidating, rather than stupid.
Then again, like most gimmicks, Austin included, the biggest part of the success or failure of it is the person playing it, although the character of Austin wasn't created as a zombie, so.....yeah.