There's really no way to explain this. SMU could've taken their extra point and taken it to OT if they somehow scored another TD. Yeah, they get the win, but why risk not sending it into OT?
The only explanation I can come up with is the Dallas Oil Booster Club told Sonny Dykes they needed the +6.5 cover.
Also if you had Memphis -6.5, it being a Fat Guy TD makes it worse.