Muay Thai fighter wasn't using his distance enough and had pretty weak and scared kicks. You can tell he was scared to throw his weight in his kicks for fear out of the boxer advancing and closing the distance, but you can't throw scared (especially when you already have the advantage starting out with the distance). He then lost confidence in his kicks (which was the best tool he had of used right) and started boxing the boxer
Not to mention, these fights aren't a 1:1 ratio in skill, so in no way does this represent Muay Thai vs. boxing ad a whole. Put Rod Tang in that bytch up against any boxer