Because the MT version is faster and less likely to get you tripped up or put off balance. Yes any time you lift your leg you make that a possibility but some techniques do it more than others. In an mma fight or Thai boxing match you have time to wear them down. It's not all about power and hitting the right spot at the right time is more important than raw power.
Member of DaJoGen MMA school under Dave Hagen and Team Chaos fight team under Denver Mangiyatan and Chris Toquero, ran out of Zanshin Martial Arts in Salem Oregon: http://www.zanshinarts.org/Home.aspx