.Net Core has come up in the shadow of .NET (Windows version) and JEE. The latter two have lost momentum and a bunch of other languages/frameworks are nibbling at market share. If it weren't for this unfortunate timing, I think .Net Core would be a lot bigger than it is.

Microsoft seems to be in this one for the long haul. I suspect .Net Core will slowly continue to gain share as time goes on, eventually reaching parity with the other 'big' players.