The backwards-compatibility with PS2 games was facilitated by the presence of the PS2 CPU and GPU in the PS3 hardware. Newer revisions of the system eliminated first the CPU and then the GPU. I think it's fair to assume this was done for cost reasons -- it's not like they removed the functionality through software update.

If you really want to play PS2 games, buy a used PS2 on Craigslist for a fraction of the cost of a PS3. Heck, you can still get them new.