Why can't you find some of the tequila I talk about on this channel, like this bottle of Cierto?
There's a lot of links in the tequila chain. You've got the agave grower, then the distillery, then the brand owner, unless the distillery owns the brand, then the importer that gets the tequila from Mexico to the US, and the distributor that buys from the importer and then sells to restaurants and stores, and then you got restaurants and stores, and then you.
That's a lot. Another issue is that each US State is essentially its own country in terms of what tequila is available. A tequila brand can only be sold in a state if they have a distributor licensed in that state.
Now, you also have to make a lot of tequila to be available across the whole country, of course. Now the big brands, like Don Julio, are available everywhere because they've got the money and the volume to put themselves everywhere. For the smaller, newer, artisanal brands, they typically have to go state by state.
While you might find a brand in California, but you won't find it next door in Arizona. And then 17 states are called "Control States", meaning that a government committee literally decides what is allowed to be sold in that state. Now that means that in most cases, new brands are kept out of the market.
This is why e-commerce is often the best bet when you're buying tequila. I've got links to three great online stores in my bio.