The big draw is probably that it's declarative, so you can define installations as code much the same way as you would do with Terraform. Instead of needing a pile of messy Ansible playbooks running custom scripts to change anything (and which can break if the target machine has an unexpected config), you'd just have one that pushes a new version of the config and runs nixos-rebuild. Rollbacks are just as easy if anything breaks. What's not to like?