Poor retailer support for titles that don't have English voice acting is a consequence of games without English voice acting selling worse.
When you say sell worse, are you referring to titles that are niche and already sell small numbers, or titles that people refuse to buy because they're not in English?
I think it's worth clarifying one of these;
Titles without English voice tracks aren't allowed to be stocked at retail, and therefore we must account for its lower store presence and far fewer sales or
fans won't purchase titles that are missing their English voice track so we can't take the risk of negotiating a higher number of copies at retail.... or both.
Since sub-only is extremely rare and has been rare for many years on store shelves, I'm much more inclined to think it's retailers jumping the gun on making absolutely sure there is an English track available, and much less of sales (that are already small and niche) taking a hammering from fans staying their wallets, since "enthusiast fans of Japanese content" are hardly going to be bothered by being told that they can only have a Japanese voice track.