Better composability and abstraction, see for example how you can configure a process in https://devenv.sh/services/ that has first-class support for development environments using Nix.
No need to use containers too, which bring cognitive/workflow overhead.
That took me about 5 seconds to type out, and now I'm in an IRB REPL slinging Ruby code, despite having never installed Ruby prior to running that command. Add another 5 seconds for nix to fetch the necessary files (which are then cached locally, so a second `nix shell` invocation will be immediate) and we're sitting at roughly 10 seconds end-to-end.
If it takes 2 whole minutes to use Docker, Docker must be pretty bad. I guess vintage things have their appeal, but apart from that I don't see why people would be so attached to such archaic, inferior technology.
No need to use containers too, which bring cognitive/workflow overhead.