MMA striking is straight up just a different game to competition TKD, K-1 rules, MT Rules, any set of kickboxing rules, or generally any set of straight striking rules.
If you see what I mean, check out Spong in his K-1 ruled fights, and then watch his stance and style in MMA.He went from being square to throw in bunches and combos to fighting in a very long stance, and only throwing generally 1 strike.
The 1 strike thing in MMA is generally because, if you are willing to throw 4 shots in a combo, you're probably gonna have to be actively moving forward on your shots. Moving forward with too much weight sets yourself up for takedowns (Grapplers are especially good at looking for your weight displacement, because that's how majority of takedowns occur). Moving forward too heavily can be bad in striking, but it can be forgiving as you won't be on your back getting Mendes elbows to the face!
Also, if you've ever experienced the difference between reacting in boxing (just punches) to kickboxing (punches AND kicks) you see how hard it would be to react to punches, kicks and takedown attempts (eg. the level dropping of Chad Mendes vs Aldo 2 where he would fakedown then unleash an uppercut on Aldo's sprawl motion).