Once the pay starts going higher, athletes might have MMA as a top option. Right now it's still not very good with a pretty low salary starting off. Unless you're a champ, or a household name, you don't get paid like other athletes do.
It takes a long time for the most part to come into MMA and become a house hold name. JBJ is a rare exception for the most part. Until that time, you could already have millions saved up in your bank account by doing the other 4 major sports.
$40,000 is a good salary for someone who is about average and has their career setup for a while. Comparing that to a below average player on the practice squad in the NFL, they make around $80,000.
That UFC money isn't even guaranteed, who knows when or if you will get injured and cannot fight, or you're opponent gets hurt at the last possible moment, something which happened to Rich Franklin recently.
I may have went totally off topic but it somewhat relates to this in the end, in order to have more athlete's even think about MMA as a career option, pay needs to be higher.