They've already explained their reasoning, I don't have to assume anything. Their reasoning is bad:

* They've fixed long-known bugs in the JVM

* They've added sub-pixel anti-aliasing and other visual rendering enhancements.

Both of these are clear improvements on the JVM, and very likely to be accepted upstream, but instead of upstreaming them, they decided to create a long-running fork and package it as their own runtime. And like all long-running forks, they're struggling to keep up.

So we are left with two choices: abandon the hope of a functional IDE that only works well on a customized runtime, or abandon the litany of performance, capability, security, and stability improvements in the JVM standard and reference JVM implementation.