Probably because most systems quickly adopted mitigations and attackers then moved back to lower-hanging fruit.

In March of 2019 there were no world wide pandemics forcing hundreds of millions to shelter in place.

The point is you vaccinate before a illness starts spreading if you can, because things spread quickly when you do not and can create quite a mess.

"I've yet to see a horse run out, so why would I bolt this barn door?"

I'm truly shocked by the comments I'm seeing here. When did so many people forget everything we've learned about security? You know what a zero-day attack is right? You know how fast those can cover the whole internet these days right? So why would you purposely leave a gaping security hole in your system to get some performance on a CPU that's probably too fast for your realistic workload already?

I've never seen reports indicating any exploit running involuntarily in the wild, ever.

That fact, however, does not validate my position.

I don't think anyone has actually bothered to a microarchitectural side channel in practice yet.