Would you like to donate money to Riot to keep those additional servers up and running?
I think you forgot that League is a free to play game, Riot isn't necessarily swimming in hundred dollar bills.

If an "NA Mexico" server was made, the population would be so low that people will just come back to the original NA server, thus wasting money and time on creating the server. Brazilian server is in bad shape, from what I hear it's mostly the PR department at fault but that's hearsay.
NA servers are in a good spot. Even Brazilians still use "mia mid/top/bot" "b" and all the other "English" acronyms on their own server. Language barriers are easy to overcome if you don't start the game hating them for not speaking your native language.

English is the national language of the vast majority of the population that plays on the NA server.

I don't include Greenland, as they are physically closer to the EUW server. The population of the entire Carribean is too small to cater to.

And then there is Mexico. Many of them do speak enough English to communicate in LoL, but refuse to. Others speak absolutely no English, and thus unable to communicate. Both are bad situations for the English-only speakers, no matter what country they are from.

Let's be honest, nobody is complaining too much about the French speaking Canadians, because they will speak in English when they need to. In fact, you've probably played with a French speaking Canadian and never knew it.

Meanwhile, you ALWAYS know when you've played with a Mexican.

I quickly Google Spanish phrases when I want someone to q-dodge. Am I a bad person?

Creo en el corazón de las tarjetas.