Maybe its a matter of function......maybe.
Perhaps the MMA style knee strike--as described above-is more in the manner of "softening up" the guy. Most of the MMA/MT are in seriously hard condition and single knee is unlikely to work. But hit them with it a lot and the effects are compounded.
Maybe its a target of opportunity--you hit what's available?
Maybe its a question of timing? I've seen guys run right into a jab and cause themselves more damage than the jab alone.
Sorry I don't have better thoughts right now. Still pretty early here.