There are a few things that are involved. The first may be that you have to keep in mind that Muay Thai evolved from rope bound fighting, where by report fights lasted very short periods of time, knock out or nothing, because they were basically barefisted. When hands were padded, this lengthened fights. I suspect that padding a part of the body devalued it. You are hitting someone with bare shin bones, or with padded hands. You can understand why the padded part might be devalued. Yes, if you rock someone, punches definitely count. But if you are just touching them stiffly, then nothing much. Punches need to have impact. Another element of this is that it is my theory that Thais and westerners map the body very differently. In the west we picture the head as the center of the self. Anything that strikes the head, even lightly, feels like it is hitting the very identity of a person. In Thailand you have a much more Old World conception of the body, something you see in traditional cultures, like those of Ancient Greece. In this view the "Gut" is a major center of self. In English we still have the vestigial belief in phrases like "gut check" or "I could feel it in my gut". For Thais, I believe, strikes to the gut, especially by kicks and knees, just FEEL like they are hitting the center of a person, more than they do in the west. Striking their vitality. In the traditional view of the body, the gut, the spleen, was a core of the self. Yes, the face also holds a symbolic sense of identity, but it is not the same as in the west.
And lastly, I suspect that because so much of Muay Thai striking is geared toward kicks (and knees) to the body, and because this centerline is very well defended, there is a kind of "capture the flag" aspect to this. If you are able to penetrate defenses, and nail the mid-section, you are really showing skill and control over the fighting space, something that was quite appreciated in the Golden Age. Control over the fighting space was the art of Muay Thai. I suspect punching people in the face (or for that matter, low kicking them, which also ends fights) was considered low-hanging fruit. These are relatively easy strikes to be had. The Muay Thai of the age was really about pushing the technical limits of attack and defense.