MAUI has had some growing pains and the rumors are that it's just a small team working on it at the moment. I think Microsoft may have bit off a bit more than they bargained for in part because they wanted to maintain a clear upgrade path for Xamarin.Forms and because they wanted to use native controls rather than just draw everything with Skia (like Flutter).
MAUI doesn't support Linux through official Microsoft channels, but there are people working on Linux support (I don't know what the state of that is at the moment).
.NET Core has been available on Linux for even longer then .NET 5. To make a long story short, Microsoft released .NET Core in 2016. Microsoft announced that .NET Core would be the future of .NET in 2019 and that .NET Framework (the old, proprietary, Windows-only .NET) would get security fixes and such, but wouldn't get any real updates. With .NET 5, Microsoft dropped the "Core" branding on .NET Core.
It's really easy to run things like .NET web services and such on Linux. You can even compile to a single binary.
Microsoft started retiring WinForms back in 2014. They open sourced it in 2018, but no one has really made adding support for non-Windows platforms a priority, especially since Microsoft put it into maintenance mode nearly a decade ago.
There are cross-platform GUI kits like AvaloniaUI and Uno Platform that some people like in addition to MAUI. Avalonia takes a Skia based approach like Flutter. Uno is a bit of a combination of Skia and some native widgets.
