Brock Lesnar failed to appear on Monday's WWE Raw and there are several reasons as to why this may have been the case. The leading theory is that Lesnar decided not to show up as he knows his contract with the company is set to expire at WrestleMania and he is trying to get a better deal out of company should they wish to re-sign him.
Another explanation is that WWE is upset with him for the photo of himself and UFC's Dana White appearing on social media during the Elimination Chamber event, so they decided not to use him for Monday's Raw.
The final theory is that WWE purposely scripted the show like this and told Lesnar not to come to Monday's Raw.
Justin Barrasso of SI.com, is also reporting that it may have been due to Lesnar's contract dates running out. That is something which isn't being widely reported and it would seem odd that WWE didn't take this into consideration heading into WrestleMania.