Part of the reason was putting into law the option to become a crown witness, famously used by Jarosław "Masa" Sokołowski.

In my view what really did them in was Poland's EU accession later on. It gave young men alternatives to joining organised crime as foot soldiers.

Of course many criminals emigrated as well, but on foreign turf they weren't nearly as successful.

Right now it's demographics, with the average age of a felon going from ~20 to ~34 between 2000 and now. These people focus on e-commerce scams.