They are not interested in ARM support / don’t have the resources for it.
There were many requests, some community efforts to make it work (https://github.com/atom/atom/issues/7822), and some Russian hacker girl seemingly got it working (https://github.com/hypersad/atom-armv7l).
However the problem for me is in the dependencies, Atoms build prerequisites (https://flight-manual.atom.io/hacking-atom/sections/hacking-on-atom-core/#building) are bloody confusing and so far refuse to install on the Pi under Raspian without conflicts, giving me the creeps and causing the build to fail…
But that is a different story… (and probably the reason why their Snap is not available on ARM, just like their other packages)