#2 No more streak
Whether the WWE Universe likes it or not, the streak was one of the only reasons The Undertaker still competed, and now that it's gone, there really isn't much point anymore. Again that probably plays back into the nothing left to prove aspect, but The Undertaker not having the steak anymore takes away a lot of the mystique surrounding his character.
The streak was the last great thing about WrestleMania and once it was gone, so was any reason The Undertaker should be in a ring. Face it! Undertaker isn't as exciting to watch without his streak on the line anymore, and while some fans still fall for his unique persona and presence, it doesn't make up for what he has lost.
In the end, The Undertaker probably harbors a lot of anger about losing his WrestleMania streak. Furthermore, he probably would have wanted one more chance to settle things with Brock Lesnar at the showcase of the immortals, but what would even be the point of that anymore?