This is a great question and I wish I had the space and time to do it justice.

Perhaps the most obvious difference between Mesoamerica and North America proper were population densities. When the Spanish arrived in Mexico, they found a land whose population density was perhaps the highest in the world at the time. Millions upon millions of people were living in an area smaller than Europe. Canada's population demographics were extremely different. Instead of massive cities, most of the new British territory was sparsely populated by European standards. The entire population of the territory occupied by the present US and Canada is almost 10x larger than Mexico, yet likely contained only as many people as Mexico alone. The rest of Mesoamerica (Guatamala and Belize to Nicaragua) would have approached central Mexico's population density. Even had the Spanish wanted to, the policy of extermination and removal pursued by British colonials would not have been practical in New Spain.

Secondly, Southern and Central Mexico's politics were extremely different than those of the Canadian groups. When the Spanish conquered Central Mexico, they were able to insert themselves into the upper levels of pre-existing social hierarchies. A long history of complex, sedentary societies in Mexico had resulted in political systems the Spanish were able to understand and manipulate to their advantage (with native help). British colonials had such no wide social systems to use. When the Spanish moved north from central Mexico into aridoamerica, an area with much more fluid social systems, their "inexorable conquest" began to more resemble a war of attrition. The strategies they used in those areas (such as the Presidio system) are much more similar to those of the British than those of the central Mexican conquistadors.

The labor systems of the colonies were also affected by the politics of their mother countries. After the conquest of Mexico, a tremendous amount of labor was needed to begin its transformation into a productive colony for Spain. In many other areas, this labor was supplied by cheap African slaves. Spain had greatly reduced access to this market because of its domination by other powers. Instead, they turned to the existing labor in Mexico to fill their shortage through the encomienda system. The politics of the Encomienda system are fascinating and I highly recommend you read up on them if you're curious about the nature of early to middle Spanish rule in the New World. The relevant point here is that the system explicitly was not slavery (although this would have been little comfort to the people exploited by it). Rather than treating natives as property to be exploited, they were instead recognized as people to be exploited. In the end, Spaniards were forced to work with local groups to extract labor and keep them (somewhat) healthy in order to work. The natives were thus able to participate in the new Mexican society, rather than marginalized and eradicated as in British territories.

However, let's fast forward to the future a bit. The year is 1821 and Mexico has just become independent from Spain. You believe in the new country of Mexico and want to announce its greatness. Do you choose a symbol representative of the distasteful Spanish rule you just rebelled against or a symbol of your glorious indigenous heritage that was destroyed by the cruel Spanish? Chances are you'd be more inclined to choose the latter. This is exactly what Mexican revolutionaries did with the Mexican Eagle so familiar on their flag. The symbol itself was a remnant of precolumbian Mexico and the near-mythological Aztec Empire. When they asserted their independence from Spain, they were simultaneously implying legitimacy descended from the Aztecs, the rightful rulers of Mexico. Something similar is occurring today in the Mexican state of Chiapas among the modern Maya.

To paraphrase, it's a complicated question. The realities of Spanish Mexico necessitated the political and social inclusion of natives groups in a way that the North American British colonies did not. The method of the Spanish conquest was also partially responsible for their mixture, as the Spanish were able to co-opt existing social structures to administrate their vast empire. The British policy of extermination and rigid social delineation of native/non-native identity was less inclusive of indigenous culture.

Modern (and semimodern) reclamations of identity did not necessarily stem from actual biological heredity. Those with the "purest blood" were still at the lowest totem of the social ladder after independence. Instead, much of the iconography and fused culture came out of a distinct Mexican identity by Mexican colonials under the European Spanish. They used connections to the Mexican past to reassert their own identities in their present.

Demographics made it unlikely that Canada would develop an Indigenous / European fusion culture.

The First Nations populations of Canada were severely reduced, largely by epidemic diseases, to a low point around 1900. In 1900 the First Nation population of Canada was 127,000, or about 2% of a total population of 5,300,000 (of which 5.1m European, 23,000 Asian, 17,000 African, and 127,000 Indigenous).

http://www.chrc-ccdp.ca/en/getBriefed/1900/population.asp

With only 2% of the population Indigenous, there was not a high enough presence for a fusion culture to develop.

(Since 1900, with the arrival of modern medicine, the Aboriginal population of Canada has grown to 1.4 million, and now makes up 4.3% of the population of Canada.)

In Mexico, Indigenous populations also got hammered in a demographic catastrophe by diseases arriving in the Colombian exchange. Estimates vary a lot, but it seems that the bottom of the demographic decline was in the mid 1600s, when the indigenous population in Mexico reached a low point of between 1.2m and 3.4m (down from pre-contact estimates between 5 and 25 m).

Denser Indigenous populations in Mexico may have enabled diseases to become endemic (by about 100 years after the Colombian exchange) instead of remaining epidemic (as they did in much less dense populations in Canada). Endemic diseases are still deadly, but do not cause such drastic population declines, as they tend to kill off children (the adults being largely survivors who have acquired immunity), and children are more rapidly replaced than adults.

http://www.hist.umn.edu/~rmccaa/mxpoprev/cambridg3.htm

As well as the demographic crash from diseases bottoming much earlier in Mexico (250 years earlier), the numbers of the non-indigenous population were much smaller. In 1650, it is estimated that there were 10,000 Mexicans of European descent, and 35,000 Mexicans of African descent. In addition, there were circa 600,000 Mexicans who were classified as “mestizo” (meaning of mixed Indigenous, European and or African descent) as well as the 1.2m to 3.4m (est.) remaining indigenous population.

With the Indigenous population of Mexico being a much higher proportion of the population at the trough of the demographic crisis, and a much higher rate of inter marriage between the populations, it is not surprising that Mexico should have formed more of a “fusion” culture than Canada did.