I never understood it
PS2 had a 300Mhz RISC chip. PS3 Cell 7 cores running at 3Ghz. Quite why they couldn't get an emulator to run with that disparity in performance between the two seemed odd. My conclusion was that it was simply a business decision. Why pay to code a decent emulator when the net result actually makes you less money becuase your customers buy 2nd hand PS2 games for a quid.