There are a huge number of Java programmers who know very little if anything outside of it, who've geared their entire careers towards learning the Java ecosystem and don't want to learn anything else.

There are also thousands of companies which are focused on Java, Java, and more Java. Many schools are also geared towards churning out more Java engineers, and people who don't yet know Java want to learn it because there are still so many Java jobs around.

That's a lot of inertia to overcome.