Despite his unrivalled charisma, his loyalty to Vince McMahon and his undisputed position as an all time Legend, Undertaker was never really able to cement himself as the "top guy" during any of his runs with the WWE. When he debuted in 1990 he was used mostly as a typical monster heel. In fact he was used as a heel for most of his early career. Even in his most prominent role in the Corporate Ministry, his role was beneath Vince McMahon (he was beneath McMahon, Austin! He was beneath McMahon all along, Austin!). When he took time off and returned as the American Bad Ass character, he was a full-on babyface. This was his first real opportunity to be the true face of the company, but unfortunately the Attitude Era was in full swing. The company was led by Stone Cold or, to a slightly lesser extent, The Rock. There's no disputing he had a major role in the company during these years, and was still a big star, but he wasn't "the guy". Then, Cena came along. An interesting point of note that helps illustrate how the Undertaker never quite made it as the Top Guy is raised by a quick look at how many times he's main evented Wrestlemania. Even with the streak in full swing, the Undertaker match has only closed out the biggest show of the year three times. First at Wrestlemania 13 against Sid, in the year when Shawn Michaels refused to drop his title to Bret Hart. His next main event was 11 years later against Edge at Wrestlemania 24, and his last was at Wrestlemania 26 in Shawn Michaels' retirement match. Of course, three Wrestlemania main events is a great achievement, and The Streak is an achievement that will never be matched, but it goes to show what a particular kind of characteristics are required to be the top guy in WWE.
Michael Palmer is a contributor at whatculture.com and thelineofbestfit.com, and he probably likes WWE slightly more than most people would call "healthy".