There's a feeling within WWE that because the company had Brock Lesnar make his re-signing announcement on ESPN earlier this week, he will be retaining the WWE World Heavyweight Championship at WrestleMania 31 tonight.
Another reason is because WWE is reportedly concerned about Roman Reigns. The feeling is that Reigns becoming the new champion tonight would be a bigger issue with the fans than having him lose his first WrestleMania main event. There's also a belief that if they put the title on him this soon, it could become a situation where it hurts him in the long run to the point that he won't be able to recover from it.
As far as tonight's main event plans go, there has been some talk that WWE made some late changes to the creative plans for the match, although what those are aren't quite clear.