#2 No suitable opponent

Sure, WWE could put The Undertaker up against several Superstars on the roster, but what would be the point? How would it further any kind of storyline and how would they cover up Undertaker's limited mobility? Unfortunately for fans, it won't further anyone, it doesn't serve any kind of purpose, and there is no real way to cover up The Deadman's limitations.
In all honesty, there is no opponent Undertaker could face at Super ShowDown that would serve any kind of real purpose. It would just be trotting out an old relic of the past, watching him get disrespected by some heel, and then hitting his signature moves before retiring to the back.
It's something WWE doesn't need right now, and will only take away from the rest of the show. It will also make any possible match at WrestleMania lose its appeal, which is something the company needs to watch out for. Doesn't Undertaker deserve better than that anyway?