Simply put, because the PS3 is too expensive to manufacture, and the PS2 chips the PS3 used for BC cost money. Removing the PS2 chips probably also allowed to make a smaller motherboard.
When Sony realized the $600 / 600 € price was not working, they scrambled to cut the production cost and the result was canning BC.
My Mario Kart Wii friend code: 2707-1866-0957