This is loosely related so forgive me if it’s not helpful.

The world has been broken up into economic zones for some markets. Using DVDs as my example. en.wikipedia.org/wiki/DVD_region_code

Mexico falls into region 4 while the USA and Canada fall into region 1.

This serves several purposes but I’ll focus on one.

This allows the publishers to sell DVDs at different prices for different regions. This is to accommodate the different buying power of the average citizen of these regions, without competing with themselves at an international level.

I suspect why you don’t see Mexico on the tag is a reflection of this concept. You’re not selling the book at an equivalent value and advertising that might cause salty customers that want the less expensive price. Or citizens of that country demanding the alternative price while abroad, complicating things.