My assumption is because the PS2 is still selling units nationwide and they are still selling units at around £80 a hit.
So by keeping the Backward compatibility in the 60gb version, they were killing their ps2 sales, not what they want to do.
It also cost them more to do it this way, so by stop selling the bigger more expensive version, people would still buy the PS2 (and hopefully in a couple of years, pay again to upgrade to the PS3..
My theory
A