Why aren't fully asynchronous circuits more prevalent?
I spent some years in a startup commercialising async design technology, so I'm familiar with the reasons:


async isn't intrinsically faster. The worst-case path delay remains the same. It's just that sometimes you get to take advantage of a faster path executing.
async has overhead of completion detection too.
Design tools. This is the really big one: there isn't really a full async "flow" of tools available to the same quality as synchronous design.
Training. You'd effectively have to retrain all your designers on the new paradigm and tools.
Risk and conservativism. So much of the industry is "produce something similar to the last one, but a bit different". This has a very high chance of working. Companies are much more reluctant to build something totally different since it has much more chance of being a total writeoff costing tens of millions of dollars.