So, why did this wave of poisonings suddenly start? I doubt that black market producers were using legitimate materials up until now.
No matter how illegitimate materials the black market producers are using, they sort of have to keep clear from certain materials. Illegal alcohol production can be a very lucrative business but having even relatively small amounts of methanol in the product can quickly destroy that business. If the customers start to drop dead or just go blind, they're not going to buy from that vendor again, furthermore the hospitals will correctly diagnose a methanol poisoning and then it shouldn't take long for competent authorities to detect the source of said methanol, which probably takes the black market producer permanently out of business. So this is one thing even criminals try to avoid.
Any time a mass methanol poisoning takes place (they're more common in India) it tends to be a result of new incompetent producers trying to enter the black market and either not realizing or not caring that their product has a fatal flaw. However banning the sale of legal hard liquor altogether seems to be a problematic measure - after all, now all the hard liquor sold in Czech Republic is surely illegal.