1. The cost of moving a standard .NET application to .NET Core. If the benefit is there, it'll happen, but for established code bases I don't see why they'd bother when standard .NET works fine.
2. On the web side of things, many of the big content management systems aren't on .NET Core yet. Umbraco is the king of CMS's on .NET, and once they pull their finger out and focus on .NET Core I think we'll see a wave of developers moving to Core.
3. For general web stuff, there isn't a solid reason for people to move from the likes of Ruby, PHP, and Python to C#/.NET Core.
4. There's still a perception that .NET Core is a WIP. That's understandable, considering .NET has been battle-tested for decades, and .NET Core is still (relatively) new to the party. There was also the csproj issues from a while back, and that set off a few people from fully switching.
5. The naming has confused everyone, so the safe option is to wait for everything to sort itself out. .NET isn't going anywhere just yet, and the switch to .NET Core won't be a huge one for those that have had to adapt to new C# and .NET libraries over the years anyway.