The main selling points for me were that it's open source and they allow you to host it yourself.
Apart from these, I really enjoy the browser addons which don't require any jumping through hoops[1] and that they provide their own Android client and you don't have to play Play Store Columbus to find a decent one. It can also be used as an autofill service which allows it to interact with other apps which is incredibly useful.
But because nothing in this world is perfect, the downsides so far are:
1. Lack of shortcuts to copy only the username or only the password and forcing me to reach for the mouse. That's really annoying.
2. With KeepassXC you could have a keyfile that you was necessary to unlock your database while Bitwarden doesn't have that option. They do provide 2FA[2] but only TOTP and email for the free version (although $10/year for the premium subscription, arguably, is not much).
1: https://keepassxc.org/docs/keepassxc-browser-migration/ 2: https://help.bitwarden.com/article/setup-two-step-login/