Visual Studio for Mac is a lie, mostly. It has nothing to do with Visual Studio, I doubt they share any code. If they do, it's very little and very recent.
VS Mac doesn't support VS extensions. And a lot of other VS things.
VS Mac doesn't support VS extensions. And a lot of other VS things.
https://news.ycombinator.com/item?id=14308754
Basically, it's not a full port, and it's probably not ever going to be, but they're trying to gradually make them more alike in terms of features.
In that regard they had to start somewhere, and as such, .NET was the obvious candidate for a first release.