An example. If you want to use .Net Core you can't use entity framework, you have to use EF Core, which doesn't yet support mapping to views or stored procs, only tables.
Now you might think that's fine, maybe you only want to map to tables. But it's the same story all over the ecosystem. Many major projects have a separate netfx and core package, and many times the core package has restricted functionality. You will almost certainly find a whole host of things you need are missing.
And then you get the fact that ASP.Net Core is dropping support for .Netfx in the next release, so if you want to use ASP.net Core you are now limited to EF core too, which means you are making a full jump and can't just go halfway first. And .netfx is now not going to support .net standard 2.1.
The list of incompatibilities and restrictions just goes on and on. I certainly can't keep the full list in my head. So right now I'm not really willing to commit to a new project of any significance on .net core because I know there will be dozens of missing dependencies or features.
You could argue this is just want it's like at the cutting edge, and the gaps will be filled in time. But .netfx is clearly already going into maintenance mode, so the push to move to .net core has already started.