Punch and low kick are easy to throw and land and require less skill. They are still very effective and can win a fight of course - Seans analogy of a street fight shows that if you were in a street fight you would use the more damaging simpler techniques that leave you less exposed.

But a Muay Thai fight isnt a street fight, the fighters are very highly conditioned and the fighters are looking to SCORE. They are not always looking to inflict damage with every technique or score a KO they are looking to unsettle, unbalance, set up, soften up, wear down as well. Often fighters that go out looking for a KO will fall behind on points as the judges will also score good defensive skills and the fighter will be open to the counter it they just go forward all the time. Kicks and knees to the body require good timing and are much harder to land.