They either needed to kill it, or make it at least as good as it was on Windows - guess they decided killing it was easier.
The number of .NET shops that have their developers on Macs has got to be quite small.
Our modern .NET Core codebases aren't much of an issue (pre core 3.1/5 are tricky without Arm support but those are easy enough to upgrade) but we have a bunch of older Framework codebases that forces them to run Windows VM's.
The equation for MS is probably that VSMac was good for those targetting the older .NET systems but with Apple going for ARM it's just too much work to get them decent (and Mono's Framework support didn't seem good enough for even a fairly simple project we tried to get running recently) and with the new C# SDK for VSCode mostly giving parity for newer projects there is fairly little reason to spend any effort on VSMac.
Care to add any detail, and reason?
I tried to use it on a Xamarin project a few years ago, and between the poor autocomplete and frequent crashes, I switched to JetBrains Rider for everything but XAML editing. There were also features that were locked behind a $250/month Enterprise subscription which other IDEs provide for free.
https://en.wikipedia.org/wiki/MonoDevelop#Visual_Studio_for_...