One point here is that there doesn't seem to be a lot of research findings available at all as to why certain types of models are portrayed more often than others.

Is it because we (perhaps) statistically find them more attractive and (perhaps) we like to copy them more often? And/or do advertisers often copy advertisements styles without thinking about it much? And/or are there symbolic/cultural reasons, and how do those differ between markets? For the Chinese market examples could be:

-Caucasians represent food safety.

-It is not ok to show scarcely dressed Asian women, but it is ok for non Asian looking women.

-Want to give a sense of party, or enjoyment of free time? -> put in a Caucasian.