There isn't as many C# developers on Linux and MacOS because the experience has been terrible for a long time and still is lacking to a smaller degree, you have the cause reversed.
> Microsoft understands, among other things, that they can't charge money for Linux tools.
It's the opposite trend going on, they embrace as much as possible Linux and port as much as they can.
Since they just focus on Azure and just milk Windows with ads and let it rot nowadays, I'm guessing it must be very profitable, otherwise they would stop.
I'm sure Azure is profitable for Microsoft. Microsoft are not even trying to sell development tools (the category of tools that would include LINQPad) to Linux devs.
They don't care about development tools period anymore, they just make most of their business on the cloud and most of it being very dependent on Linux.
Half of Stack Overflow "developers" might not use Windows, but the relevant population to measure is C# developers.
Assuming as many Linux C# developers will buy the thing as Windows or MacOS developers do is an obvious mistake.
> Even Microsoft themselves understood this.
Microsoft understands, among other things, that they can't charge money for Linux tools.