Business sense....
The rom that basically allows you to select an 'other os' costs Sony a small amount of $
They estimated the cost of including it in all their new PS3 slim's which still cost Sony more to produce than they sell for, was simply untenable to the amount of users who actually use Linux on their machine.
They saw that the vast majority want the PS3 to come down in price. Compare that to the fractional minority who will mess around with Linux on the PS3 before realizing that it's actually crappy on the PS3 and then delete it to make more room back on their PS3 HDD's was too negligible to justify the continued expense.
For similar reasons Sony determined that there as no more need for PS2 backward compatibility. Their desire is to drive forward PS3 software sales and recoup $$$$ on the hardware losses.
Just business.