Yes, like any illegal drug, the extent of production will depend on overall demand, the ease and availability of the skills, materials and equipment required to produce it, and the difference between the local unit production cost versus unit market sale price at intended market location. The second and third factors vary depending on location due to differences in consumer tastes across cultures and locations, differences in local laws, penalties and enforcement or the nearby presence of specific industries (from which to order/steal supplies and equipment).
Overall in this case, I think it is due to the drug's mechanism of action. Qualudes appear to positively modulate neuronal GABA [an inhibitory neurotransmitter] receptor activity, producing a sedative/hypnotic effect. I would guess that they act similarly to barbiturates (such as pentobarbital) and benzodiazepines (such as valium/diazepam) which both also stimulate GABA transmission. Barbs, whilst still available and in use in some places are generally shunned by almost all medics and even a lot of recreational drug users due to their extremely high potential for fatal overdose (so much so that they are the drug class of choice for painless euthanasia of both animals and humans- due to either terminal illness or crime). In their place, benzos (which are in widespread medical use- and therefore easyish to obtain) are much more common and are much safer than barbs (although still have high potential for fatal overdose, especially when mixed with other sedative drugs such as alcohol and opiates [eg heroin, methadone and codeine]). However benzos were only discovered and became available fairly recently. Previously, barbs and other dangerous sedatives were in high use and demand, resulting in illicit production.
Therefore I would guess that the availibilty of newer, better (from both consumer and supplies point of view) more cost efficient drugs meant that quaaludes faded from most illegal drugs markets. The exceptions where quaaludes remain popular probably exist due to some local quirk involving one of the factors outlined at the start of the post.
P.s. Apologies for the extended essay.
TL;DR: new, better (from both the supplier and consumer's point of view) drugs have taken its place in most illegal drug markets. However it can and is manufactured if desired.