It's more that Visual Studio is a massive project that was built around running on Windows. It would be a huge undertaking to bring it over to another OS and probably not add nearly enough value. The only reason macOS has "Visual Studio" at all is because Microsoft bought out Xamarin and rebranded their Xamarin Studio product.
Yup it would just not be worth it to bring real VS to macOS. Then why decrease its brand value by marketing a crappy product like Xamarin (which I think has always been bad, and I honestly don't know why MS bothered to buy it) as if it was VS? Just call it Xamarin, or VS "lite", or anything, but not VS as it is not VS.
Consistent branding. Visual Studio is the name they're using for all their code editing software. I don't think it's a good idea given the confusion it causes when people talk about the various programs, but they are distinctly named Visual Studio, Visual Studio Code, and Visual Studio for Mac.
Microsoft has never been good at naming things. They made that mess of Windows naming at multiple points and they are still making a mess with the Xbox brand.
