This isn't really true. Mono functions as a complete replacement for the ".NET Framework" - something that can be used to run any .NET app, including "legacy" apps targeting old ".NET Framework" versions, on any supported platform, even when the app was built to target Windows.
dotnet/runtime is intended to run more modern applications that target ".NET Core" - basically, stuff that's cross-platform on purpose.
There are tons of subtle differences relating to these goals but also some glaringly obvious ones, like mono having an implementation of Windows.Forms.
> hence the lack of commits and contributions
Microsoft have been actively forcing contributors out of mono/mono and into the dotnet/runtime repo for several years now, while Wine kept a weird halfway fork at https://gitlab.winehq.org/wine-mono/mono . Formally transferring `mono/mono` and the Mono name over to Wine will in theory allow `mono` to more effectively accept code which works to improve legacy .NET Framework support for compatibility reasons, while dotnet/runtime can continue to evolve as the way to run intentionally targeted .NET Core code.