The problem is that the cat's they were buying were probably not made to factory quality specs, that's why the state of California changed the requirements. The so called "High-Flow" replacements were the worst as they usually had even less catalyzing material in them (Rhodium, Platinum and Palladium, all very expensive metals). Two years of use were usually enough to cause enough wear and erosion to prevent them working properly.

Here's some links on this subject:
http://www.arb.ca.gov/msprog/aftermktcat/amcatq&a.pdf

http://www.bar.ca.gov/80_BARResources/ftp/pdfdocs/ARSC-summer08.pdf

http://www.arb.ca.gov/msprog/aftermktcat/aftermktcat.htm

The notes from the hearings in the last link are interesting.

Hope this adds to the discussion!