Probably because noone wants to user rizin when you can use radare2. :-)

I'm still fuzzy on the difference between the two, would you mind to elaborate?

There are many different technical differences that accumulated over time - we save projects as a state snapshot, not a sequence of commands[1], we save types as semantically connected structures in a database that is guaranteed to be consistent[2], use better stack tracking for arguments and variables[3], not SP/BP/whatever, slowly migrate to a new generation of IL - RzIL instead of ESIL[4], provide standard libraries signatures out of the box in the FLIRT format[5], switched to a new way of parsing and processing commands[6], provide basefind, and many other small differences.

[1] https://rizin.re/posts/introducing-projects/

[2] https://github.com/rizinorg/rizin/tree/dev/librz/type

[3] https://github.com/rizinorg/rizin/releases/tag/v0.5.0

[4] https://github.com/rizinorg/rizin/blob/dev/doc/rzil.md

[5] https://github.com/rizinorg/sigdb

[6] https://rizin.re/posts/rzshell/

What about x64dbg? Is that considered the standard on Windows?

Other options are typically more fully-baked and conventional.