The answer would require more in-depth explanation than I could possibly give here as one could take into account the immense number of centuries of the development of both populations going back to their very long and immensely different histories in England and Spain, however, there is one difference I will highlight here, even though, given my political views, I do not like the sound of it, nevertheless, I cannot believe other than that it is true. First of all, one thing that most monarchists will probably agree on is that stability for a country is extremely important. Human beings can adapt to a wide range of extreme hardships and difficulties so long as there is stability. You can put considerable hurdles in the path of humanity and so long as those hurdles are fixed, people will usually find a way over or around them. Stability matters and, I think, it matters a great deal that the history of the United States, other than one extremely brutal four-year period of mass fratricide, has been a very stable country whereas Mexico has not. In fact, the longest period of sustained stability in Mexico was, I hate to say, probably the better part of a century the country spent as a socialist dictatorship under the PRI and that stability was based on government force, brutal violence, deception and corrupt agreements with organized crime. Not an ideal sort of stability by any means, nor one that could possibly be sustained (as it wasn't).
|Tories land in Canada
Because the Tories left the United States, the population that remained was dominated by the other two-thirds (as John Adams famously estimated it) who were Patriots or had no problem with the Patriots. Although there were certainly plenty of these early Americans disagreed about, by and large, they all agreed on the fundamental foundations of their new country such as republicanism, egalitarianism (as understood at the time) and a level of democracy. They may have argued, as the Federalist and Anti-Federalist factions emerged, about exactly what sort of constitutional republic they were to have, but that they were to have a constitutional republic rather than a constitutional monarchy with an established church and titled aristocracy was beyond question. After all, everyone who passionately favored those things had left the country, doubtless shaking the dust from their shoes as they came aboard ship. I think this played a very critical part in why the United States was as stable as it was, for as long as it was, until the states' rights and slavery issue boiled over, resulting in a unity that was, certainly in the immediate aftermath, based on armed force by overwhelming military might. Mexico, on the other hand, had a very different set of circumstances.
|The Embrace of Acatempan
The result was the success of the anti-Spanish forces, the independence of Mexico and the short-lived first Mexican Empire with General Iturbide as Emperor Agustin I. It was short-lived, however, because the republicans were still there and soon fought to overthrow and ultimately kill their Emperor. Likewise, when Guadalupe Victoria assumed office as the first President of Mexico, those who had favored the empire were still on hand. They could not, as the Tories had done, simply move in a mass exodus to some other part of the Spanish-speaking world because they were, as far as the Spanish were concerned, just as much traitors as their former republican comrades had been. Iturbide, for example, had gone into exile in England rather than Cuba or Spain itself because the Spanish authorities would have executed him for treason. The Tories in America had, as their most fundamental principle, their loyalty to the British Crown and King George III whereas in Mexico, the monarchists had broken away from the wider, global Spanish empire in favor of having an empire of their own so that, even had they desired to, there was no way they could turn back from the path they started down with Iturbide.
This is, of course, simply my opinion, bolstered by historical facts but some may draw other conclusions from the histories of the two countries. However, it seems to me that this played a major part in why the U.S. was so much more stable compared to Mexico. I do not doubt that, in this day and age, many will disagree. After all, if the argument I have made is correct, it would be rather in defiance of the current popular slogan that "diversity is a strength". However, I do not see the historical record as supporting such a notion. The outbreak of the Wars of Religion in France or the Thirty Years War in Germany after the birth of Protestantism brought about more "diversity" of views on Christianity would seem to support such a position as would the lack of such wars in places like Spain, Italy or even Russia, which lacked such diversity. To me it seems a rather obvious point of view that one cannot have a great deal of conflict over politics or religious if everyone holds essentially the same fundamental beliefs about such things for the same reason that there are no race riots in Japan or clashes between Christians and Muslims in Saudi Arabia. Diversity, I would think, tends to necessitate divisions and few things can cause instability like divisions regarding something so fundamental as the very form of government one is to submit to.