Because the MT version is faster and less likely to get you tripped up or put off balance.
How do you figger? The MT version's execution describes a right angle. I.e.: First it comes up, then it traverses to the centerline. The front knee strike moves up and in simultaneously in an oblique line. The roundhouse moves up and in in a curved, rising, simultaneous movement. These last two are faster than the two-step trajectory of the MT technique.
As to getting tripped or off balanced, in the MT knee strike the leg hangs more or less at right angle to the thigh. There's a gap between the thigh and the leg. In the front and roundhouse versions, the leg is brought in tight against the thigh. The gap and the added slowness of the MT version makes for greater ease of it being scoop-blocked, the fighter then being unbalanced and swept/leg-reaped.
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.
OK, but in any
fight (particularly in the street) you ideally want the opponent down and out as quickly as possible. And the more power you can get into effective techniques, the more probably you'll succeed. So why not opt for the stronger, faster effective strikes?
BTW, I've been using the term "Karate knee strikes" just as a convenient way to differentiate the front knee strike as well as the roundhouse variant. In fact, it's a misnomer as they're not exclusive to Karate. Both the front and roundhouse are pretty much universal to all hand-and-foot fighting arts, including MT/MMA. So it needs to be borne in mind that this makes it pretty evident that MT/MMA doesn't consider the front and roundhouse versions ineffective. So why go bother with this third, weaker variant?