Part of it is cultural while another part is economic.

Culturally, Canada was part of the British Empire well into the 20th century. This meant that having relations with Canada was having relations with the British Empire. Outside of the War of 1812, the British Empire typically negotiated with the USA with respect that the USA could project at least the same kind of land power in the Americas as the British Empire could.

Culturally, Mexico was a successor nation to the Spanish Empire. Mexico didn’t have the level of protection that Canada did nor did it have similar government institutions as the USA had. There was also a difference in approach to colonization between the two nations; the USA engaged in settler colonialism using white Europeans while Mexico had a large indigenous population incorporated into Mexican society.

Economically, Canada has a per capita national wealth on par with the USA while Mexico has lagged behind. This has led to a lot of migration from Mexico to the USA and Mexico being used as a low cost labor source.

One part of this is history.

Canada and the US were British colonies; Mexico was a Spanish colony.

When some of the British colonies declared independence, they still had to trade with the colonies that hadn’t. People had relatives on both sides, the postal systems were integrated, indigenous people were mistreated in the same manner, and the list goes on. Culturally, the two remained very similar while the political systems differed.

Stuff coming from England often ended up in Toronto or New York; both of these cities became hubs of publication.

This is the way the relationship stayed pretty much up until NAFTA in the 1990s; books had already had over a century of being published in Toronto and New York for distribution across English North America.

Mexico had a different history, and a different relationship with California, Arizona, New Mexico and Texas. Instead of Mexico being a route for culture and European goods to enter the US, it was a source of cheap labor once slavery was abolished.

Unlike Canada where the most influential Canadians lived right along the border, in Mexico the influential Mexicans lived further south, with itinerant workers living along the border.

NAFTA changed the balance of trade somewhat, but it didn’t change the already established cultural norms or the places people lived.

Because we dont usually buy books in english?

But there are several products with boxes written in both languages.

From an outsiders perspective, from what I can tell it’s just racism

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. https://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.