Stack Overflow uses NET Core. I think the reason you don't see it in the wild as much as is because NET Core is pretty new. Stack Overflow migrated in late 2018. That's really about when NET Core became mature enough to use in production for new development, never mind legacy. Most companies feel no pressure to use NET Core - their huge amount of legacy software still works and can be maintained on NET Framework. Not to mention Windows-based tech like WPF and WinForms only just got support with NET Core 3 which was released 7 months ago. Companies with 'unrespected cost centers' as noted are especially conservative.