My perspective on it is that Lesnar didn't want to live with regrets, and wanted to at least give football a shot while he was still young.
MMA was probably him not wanting to turn tail back to the WWE too quickly. But, here we are.
Remember, Brock didn't play organized football from the time he was 17 until he was in his mid 20's. He made it to the last days of training camp before getting cut. Not bad at all for a guy trying out for a pro team.
And he was supposed to come back to the WWE in 2005 and put Batista over at Summerslam that year. Vince decided to stroke his ego and make Lesnar start from the bottom up. Lesnar was willing to come back after the failed NFL career, but then that shit started and off to MMA land Lesnar went.