#2 He only appears twice a year
One of the great things about the Undertaker is his illusiveness. When he turns up it's a big deal because he hasn't been overexposed to the WWE Universe. He carries a mystical aura about him that creates spectacular television.
If ‘Taker were to win the Rumble and then win whichever title he goes for, he would be appearing more often than he usually does, thus reducing his mystique. If you reduce his mystique then you diminish the appeal of the Undertaker.
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If ‘Taker was to win the Rumble it would be a mistake, but if he did have a title opportunity he should put over the Champion and ghost until next year. But we all know that that's unlikely to happen, the Undertaker is one of the most protected wrestlers in WWE history.
The Deadman has won over 70% of his matches in the ring, and of course 95% at Wrestlemania. If ‘Taker wins the Rumble, he's winning the title, and that's what nobody wants to see.