Many things are a lot less illegal in some places, you know, without dramatic increases in use.

Those other things are in different circumstances and are other things. In general, making something illegal is... probably going to reduce how many people do it. Odd, I know. (If modafinil had been invented 200 years ago and had never been regulated, its usage in the general population would probably be similar to caffeine and nicotine, which is a much higher figure than its actual current usage.)

Expensive, like, for high performers?

Prescription modafinil is something like $300 a month or $3.6k a year. It's worth it, especially for high performers in the highest-wage countries in the world like the USA, but it's still a nontrivial financial expense on top of the reputational problems, unease with the concept, and worries about the legal risk and unknown unknowns.