It’s a two fold question.

First; the neighboring Tarascan civilization had metal working. In all of meso-American the western side of Mexico is the only part where we find metallurgy being practiced locally. They likely imported this know from South America at some point.

It had been present in West Mexico for more than a thousand years. So, why didn’t the rest of Meso-America adopt it?

We don’t know but we can make some logical guesses.

The first is that there were no known sources of copper. Early metal working derived as a result of pottery. The process of glazing and finishing pottery can produce copper slag. So its an easy progression that pottery leads to metalworking.

Problem is that there isn’t a lot of copper to be found in the soil or pottery styles of the region so they never made the leap.

But they were adjacent to metal workers for centuries. The Aztec were even at war with the Tarascans.

And they weren’t losing.

So here we come to obsidian. Obsidian is a useful rock. Its brittle but it can hold a very sharp edge. It’ll break sure… but so what? Just get more obsidian and make a new edge.

We see a similar pattern int he near east. There were groups slow to adopt metal tools because stone tools are simple. You didn’t need an artisan to make or fix them. They’re cheaper. Anyone can make a basic stone tool.

And that’s probably why the Aztecs didn’t switch to bronze. Obsidian’s general ease of use and practicality was more valuable to them. They didn’t see the advantages si they didn’t adopt copper. Don’t adopt copper you don’t get bronze. Iron takes more skill to work and experience working iron leads to steel.

This is a fairly consistent global pattern but it was stalled in Meso-American by a lack of metal sources and the many uses and ease of use of obsidian.