Vaultwarden is a very lean implementation of Bitwarden but if you want to look into an alternative to the Bitwarden ecosystem, I recommend - AliasVault https://github.com/aliasvault/aliasvault - check it out!
Probably because there is no need to fork until you have to. Why do it prematurely and have to keep it up to date when you can just do it when it is needed?
It's Bitwarden's game to lose. Forking is easy enough that there's no great need to pre-emptively fork.