I suspect it's because voltage boosting circuits became easier and cheaper to install. The components are smaller and run from a lower voltage so no need to boost the voltage. Also, AA and AAA batteries are ubiquitous and much less expensive than 9V batteries. And nobody likes them.

For myself, I have a couple spare 9V batteries for a specific piece of equipment (cable tracer) that I use only very rarely, but otherwise those batteries are out of my life.