Microsoft has had dotnet-core for awhile. If you are running production dotnet loads (eg a C# app), you’ve probably been using those Linux containers for awhile. This doesn’t surprise me; they usually aren’t interested in maintaining an open version of software they have more restrictive licenses for. Enterprises will continue to use dotnet-core and Microsoft will probably do something to shoot mono in the foot in a few years.
The reason to use mono over dotnet is political. This is stirring up some really old shit; I expect a continuation of that shit now. Mono is currently MIT as is dotnet core. Who knows what direction each project will go now? MS has a history of fucking with licenses and Wine uses copyleft setups.