In 1990 I don't think SSA was broadly considered to be the basis of a good optimizing compiler the way it is today. So the "author of a flagship optimizing compiler" would be gambling on an unknown that looked good in a couple new papers. The Cytron paper on how to efficiently compute it came out in 1991. So SSA was still a WIP.

It wasn't really until the late 1990s IMO, that SSA became widespread and into the 2000s when it became the standard.

The safe TSA papers from 2000-2001 were the papers that showed that SSA was practical in a JIT environment. The fact that this approach was adopted less than 5 years after those papers come out suggest that compiler authors are in fact reading research papers.

Rearchitecting a production JIT to use a different IR for theoretical gains when nobody's demonstrated those gains on a toy JIT is high risk and demonstrating those gains on a toy JIT is research, not development.

If you just want to write a compiler for fun, I can understand not reading up on state of the art theory beforehand. And then it's accidentally actually useful to other people and it's too far into development to change the fundamental architecture.