> Same as ARM nodes a few years ago: cheaper, denser, widely available – still not the default choice.

Because ARM nodes save you a bit of money (they're cheaper but a bit slower, maybe you end up saving 20-30%) but the move isn't trivial for many tech stacks. When you try it you find you have some Python dependency with a C bit that doesn't have an arm wheel, or your browser automation can't run Chrome on those nodes (there isn't a linux/arm64 build of it). If you're using Go you can cross-compile, although it will likely fail without explicitly disabling cgo, and do you know what the consequences of that are?

Basically it very often ended up being more trouble than it was worth.