.NET Core also doesn't run in all platforms where there is a Java implementation and not everyone is happy to rewrite their .NET Framework into Core, while Microsoft is in mix of leaving again stuff behind like the ongoing discussions about CoreRT, .NET Native, Project Reunion, MAUI vs Blazor vs WPF vs Forms show.
The problem with .NET is that Microsoft have a proven history of seeing the next shiny over the hill and dropping support for existing frameworks and libs while the "new and improved" framework is rolled out. See ! This way is better! Well, until the next better way comes up within a few years.
I know of teams who have moved to Java simply to avoid the churn. Many folks prefer the boring but stable Java ecosystem.
I jumped ship to .NET after Oracle changed their licencing. I don't like them as a vendor and don't want to be tied to them.
Java will still live to a pretty ripe old age but it does feel like it's a little over the hill nowadays.