I mean, there has been a couple dosen surveys made by the community over the last few years, using different methods etc.
I have made a couple of surveys on this forum and on the subreddit, and so have others, i never saw anyone say mexico before it happened.
there was this post i made around the launch of mexico:
IDK i think 36% for europe, 30% for Asia and just 9% for new world nation seems to me like it indicates average person probably want asian or european DLC more.
Before the, now cancelled, DLC had its factions revealed i made another poll:
The poll shifted a bit after the faction reveal but it is still relatively similar to how it looked before hand. Now obviously people where asked what they beleived it would be, but most took it as what they wanted it to be.
Again we see massive demand for European and Asian civs, and again about a 9% wish rate for new world nations.
Then there is this poll made before Mexican and TAR DLC was announced:
I really don’t see anyone asking for Mexico.
On reddit it was from what i recall a similar story, a poll i made asking specifically about civs:
https://www.reddit.com/r/aoe3/comments/texolp/what_civilization_do_you_want_to_be_next/
Obviously i do get power over what options are added but i still think it reasonably shows a pattern - Italy, Poland and Denmark gets 40% of the votes. Korea and Persians get 33% of the votes.
There is this poll someone else made that shows more vote for Argentina and Brazil:
https://www.reddit.com/r/aoe3/comments/ws280y/from_the_following_wich_civs_would_you_like_to_be/
But it still marks below Poland combo and Asian options.
A perhaps interesting poll is this in regards to canada:
https://www.reddit.com/r/aoe3/comments/rag28e/how_would_you_react_to_the_next_civ_being_canada/
The way i would read it is that a lot of people aren’t too picky with the civs and would be happy with whatever they get. This is the sort of poll i could point to as an explanation for the US and Mexicos popularity - because no one when asked to prioritise usually says Canada, and yet a lot of people still said they would be fine with it.
Maybe there are some polls out there you could point to with vastly different conclusions but i just haven’t seen them. I would hazard a guess that south american civs are more popular in south america, but well i dont speak spanish so i cant say a lot about it.
Will point out though that if there was ever a popular place to say you want Canada then it is the steam forums, though even there most people will tell you that you are silly.