Software has a Linux version and it only comes as a .deb
Depends on a load of packages exclusive to Ubuntu and installing it on stock Debian is bloody impossible
Yeah I remember when Steam came to Linux. Never got it to work. I hope it is better nowadays? (Sadly can't check. My only pure Linux box at the moment is a Raspberry Pi, I don't expect Steam to run on that either)
You can get Steam on just about any distro, for years at this point. And there's always Flatpak for these cases too although for Steam I recommend native packages.