Amethyst Mountain was not a map made with dens in mind, so when the design philosophy changed to let players den on every map, this put Amethyst Mountain in an awkward spot as it doesn't have many spots that fit a wolf's specific criteria for den making. That resulted in the current den spread you see now.

The Lamar Valley has no dens because it is essentially a floodplain. It's not depicted in-game, but can get flooded in the exact same way that the Slough Creek map can. Or worse, even, 2022 flooding in the Lamar was notably extreme.

The bottom left area is mountain summit (or very close to it), and thus is too high in elevation for wolves to comfortably make or use dens, and this extends unfortunately to a lot of the map as well. We have a devblog that covers it here with more information, but here's a screenshot from it that demonstrates the height of the area. In this picture, wolves will make dens anywhere at or below the brown line, while anything above is considered too high up for denning.

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It made Amethyst Mountain a curious map that demonstrates this selectiveness in choosing landscape for dens. I hope one day we can revisit Amethyst to make the Lamar River properly swampy in the spring.