> So, how did this happen? What were the Mayans doing that kept their culture so homogenous over such a long period, that clearly wasn't being done by Teotihuacan or the Toltecs in the west whose empires seem to have faded and left almost no mark on the cultures that follow?

In short, it didn't.

There's a couple key pieces of information to keep in mind here:

Maya is an exonym for most people under its umbrella, not an endonym. This means it is a foreign name, not a local name. It is debatably an endonym for part of the northern Yucatan Peninsula that later expanded during the Caste War in the late 1840s - late 1910s, but this was a long and complicated process. Either way, it was still only primarily used by people from the northern and western Yucatan until significantly recently.

• If you want to read more on this, I would recommend looking into the chapter Maya Ethnogenesis and Group Identity in Yucatán, 1500–1900 in the book The Only True People. If you need help finding this let me know, but it's fairly accessible online.

What prompted the greater expansion of the "Maya" label as a deliberately used label by the people themselves (and still only in a partial form) is essentially the Guatemalan Genocide. The Silent Holocaust, Maya Genocide, there's a couple names and you may or may not have heard of it prior.

The broad and short of it was a three and a half decade long insurgency war in Guatemala, wherein the military government very actively and disproportionately targeted indigenous citizens, for a number of reasons I'm not going to go into here, sorry.

Regardless, in the realm of 30,000 - 150,000 civilians were massacred very brutally by the Guatemalan military government, and in the aftermath the indigenous groups of northern Guatemala sought international solidarity both for aid purposes, and also as a defense mechanism to prevent a second genocide. It is not, particularly, a symbol of cultural unity.

• This is, by the way, a pretty brief explanation because the Guatemalan Genocide is mostly outside my area of focus, sorry!

Now, to be clear, much of the peoples considered "Maya" do have common cultural traits - but many of those are regional, and they also partially apply to, for example, the Xinca, or the Lenca, and so on. What's considered the Maya region has always been very culturally diverse, politically disunited, and similarities are more in the sense of broad regional cultural trends rather than supreme unity.

• As a note, Yucatec and Q'eqchi' are basically not mutually intelligible at all - they're part of totally separate branches of the same family, and the family is about as internally diverse as the European branch of the Indo-European language family. Q'eqchi' is more mutually intelligible with the Quichean subbranch of the southern Mayan languages, but not very.

There is an illusion of "Maya Unity" that sometimes get projected backwards - but it is an illusion, and primarily due to a lack of detail. In the Classic period (300 - 950 AD), this is partially because of the style of records not being particularly conducive to it. In the Postclassic period, it is due to the enduring bias of both the general public and many academics, of viewing the Postclassic Maya as some "degenerated" or "lesser" version of a past culture, and thus equally unworthy of greater attention. But over the last four decades, there's been a significant reevaluation, and we've come to understand the diversity, change, and accomplishments of these people to a much greater degree than before.

• If you want to read more on this, I would recommend It Depends on How We Look at Things: New Perspectives on the Postclassic Period in the Northern Maya Lowlands. It is pretty much exclusively focused on the Northern Yucatan, but is a solid overview. Once again, ask me if you need help finding it!

You should ask this on r/askhistorians to get a proper answer

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Isn't the western region more mountainous and the eastern region predominantly flat lowlands? I imagine the great difference in geography had a great impact on cultural diffusion/assimilation, with groups in the highlands more isolated from each other - hence maintaining more cultural/linguistic diversity.

One part of it is that the north and west probably saw more influxes of migrants that increased the diversity of cultures. That would result in more difference than Mayans slowly diverging from a common source.

It's a geographic phenomenon. Central America forms a physical bottle neck and as the region filled up with it's first group of linguistic arrivals there wasn't any room for new comers. They had to settle to the west and north where Mexico opens up. That's one theory!