> Maybe it's Oracle releasing new Java versions every two years now, and being more aggressive about end-of-life'ing the prior versions?

This is a big part of it.

If you want support for older JDKs, you have to pay Red Hat for OpenJDK support. Which isn't even "real" Java, in the eyes of many companies.

Add in the clamour of major libraries and platforms pushing everyone to go to Java 8 so they can slash great chunks of boilerplate code and it's a smooth ride downhill.