"hasn't been done yet"
Here's my guess. Commercial molds are expensive and the market for non-Staunton plastic sets is likely limited.
I stumbled across someone's page recently on a STL sharing site where he had STLs of many historical chess set designs. 3-D printing (possibly from a service) a historic design may be a more approachable alternative if you want a plastic Soviet design. If I can find the page again, I'll add a link. If he doesn't have an STL yet, maybe he'd be interested in a commission.
I know there are plenty vintage Soviet sets, but I believe most of them aren’t heavily weighted. Are there any modern Russian sets in plastic? I’m not finding any…
I can’t be the only one who would appreciate a triple-weighted plastic Baku set, right?
Or a heavily weighted plastic Botvinnik–Flohr I.
Or a plastic Botvinnik–Flohr II…
Or what about a plastic Dubrovnik? House of Staunton offers a plastic Zagreb, but why not a plastic 1950s Dubrovnik?
Or with the increased popularity of Mordovian sets (of ethically questionable provenance considering they were made in Gulags), why not produce a plastic version?
Heck, why not a cute little Valdai (Nobles) as an analysis set?
Or an Averbakh? A Smyslov? An Egyptian? A Tal set? A Ferocious knight (Kultsport) set in plastic would be wild!
Am I the only one who would get a kick out of some plastic Soviet reproductions? Doesn’t this constitute a gap in the market? I’m so confused why it hasn’t been done yet.