#1 Risk of injuries
The Undertaker isn't what he used to be.
Not only is that evident by the extremely slow pace he has in the ring now, but also by the limited amount of damage he can take during a match. In fact, it has gotten so bad that Taker's opponent is often relegated to punches, rest holds, and taking The Deadman's finisher throughout the entire contest.
Of course, a lot of that has to do with The Undertaker's previous injuries and the risk of further injuries happening in the ring, but it's gotten to a point where he shouldn't be in the ring in the first place.
Furthermore, with a laundry list of injuries including torn pecs, broken hips, both eye sockets crushed, and 20 orthopedic surgeries, he should be nowhere near a ring at this point in his life.
In the end, this is going to probably leave a lot of the WWE Universe resentful that they won't be able to see the Undertaker one more time, but it's probably for the greater good. In all honesty, Undertaker just doesn't possess the same magic and mystique he did, which makes trying to trot him out to recapture it all the more pointless.
It's not there anymore!