The catalytic converters are a "cyclical economy". A demand for used converters existed because they suddenly disappear from older vehicles; a new replacement is prohibitively expensive.
Plus the precious metals value is a strong incentive. The market for converters is driven by their platinum / palladium / rhodium content. The authorities could regulate sale of said metals, but that would be a political nightmare ...
So many factors involved, but the main culprits seem to be related to the drug trade; desparate and not normal minded people usually ...