... and a similar experiment in predominantly Han Chinese China likely results in the same thing biased towards Han Chinese, In Thailand towards Thais, and in Cambodia towards Khmer people.
My GF lived in China half a decade, and she had trouble with tailored clothes, because they could never accept her southern-European proportions.
The idea that the most common size should be the most common group is just business 101 (build for your customers).
Assuming their market is primarily in the US, then 72% of their potential customers are white, 12% are black, 5% asian, and the rest something else [1].
That's for the US as a whole. I'd expect the subset of the US population that will buy online custom dress shirts tends toward the more affluent end of the population, where the skew is even more heavily toward white.
So the default option is the one that is best for the majority (by a large margin) of their customers. I don't see what is interesting about that.
[1] https://en.wikipedia.org/wiki/Demography_of_the_United_State...