The problem as I see it
Basically I think the problem is perception, and certainly it is for me.
People think of the .Net Framework - that's what all the marketing hype has been around for years, and it's the proven, tried-and-tested, used in production, well known, considered stable and usable part of .Net. It's also considered tied to Windows and Visual Studio.
While .Net Core isn't as well known, and if anyone has heard of it then they think of it like "oh cute an open source version of .Net, how experimental. Come back in a few years when it's actually been used in production"
It isn't 2005 any more, yes. But Microsoft doesn't do marketing as well as other people - isn't Swift newer? And I'd imagine it already has a better takeup than .Net Core.