Because .NET Core even with the .NET Standard 2.0 only covers part of the APIs for CLI and Web applications, and only targets GNU/Linux, OS X and Windows.

Java is a complete platform, not only CLI and Web, has application servers like Websphere that offer a full server OS abstraction across the cluster and run in several flavours of enterprise grade UNIX and mainframes, not only GNU/Linux.

Also every database worth using in the enterprise space has JDBC drivers, whereas good luck getting ADO.NET drivers or mainframe connectors for .NET Core on those platforms.

And many of the .NET libraries don't run out of the box on either Mono or .NET Core, so unless they get ported, they won't be available.

Oh, and as far as I am aware you still don't get performance monitoring tooling like the JetBrain products talking to .NET Core.

Also they have legions of replaceable programmers used to Java, unless the IT department changes their infrastructure and spends money in trainings, it won't happen, even if .NET Core already had feature parity with Java.