250+ days a year on the road and all the wear and tear on your body is a big incentive to leave 'wrestling' and go mma.
Lesnar is a smart guy, he knows that wrestling will always be there so why not try your hand at other things. NFL didn't work out but I am sure that he got paid, the practice squad guys make more than the top bodybuilders and more than most wrestlers.
in MMA he will fight 2/3 times per year and make good money..also if he is successfull he will be able to negotiate with the WWE from a position of strength.
also to me (I have no proof of this) the WWE seems to be dying a slow death where as MMA has taken off. Vince McMahon is a smart guy and is probably trying to capture some of MMA's fame so I expect him to try and start a new MMA federation...so he might throw big money at Lesnar