They don’t change it from the default because G1 isn’t terrible and Java has a long history of ensuring backwards compatibility - not just in APIs, but also behavior, warts and all.

If they changed the default GC, a lot of folks would freak out, even if it was objectively better in nearly every situation. Because someone, somewhere is relying on some weird bug in G1, or whatever and now their software behaves differently, and it’s a problem.

Give it a couple more major revs, and it might still happen. G1 became the default in what, Java 9?

Naturally there are tradeoffs, for example the amount of infrastruture they need only justifies when one is working at such scale, for example ZGC requires large pages and makes use of NUMA if available.