The main problem is that many of these English congregations in Mexico don't need to exist in the first place. Some of them may have 50 people attending, and every single one of them speaks Spanish fluently. Or the only ones that don't shipped themselves in to 'serve where the need is great.' Problem is, there are practically zero native English speakers in their territory. Outside of a few retirement spots and resort areas, there are very few native English speakers in Mexico.
Just about all of these congregations exist due to an artificial 'need' that a few local witnesses (or transplants) have dreamed up.