That part of The Undertaker’s WWF/E career was very short in comparison to his much longer (and more memorable) career as The Deadman/Phenom. A lot of things that made Taker great are from before and after the biker gimmick : The supernatural powers with the urn, being impervious to pain and can never be buried alive or set on fire, the history with Kane, Mankind getting thrown off the Hell in a Cell at King Of The Ring in 1998, even his undefeated streak at WrestleMania (before Brock Lesnar ended it in 2014) etc. So why do they want him back as the biker again ? It'll be too obvious that he got older, with The Deadman gimmick he can hide it.