"Due to the high ratings of American Dad,(approx. 15 million, an 8 million drop off from The Simpsons) fox is changing its time slot from 9:30 P.M. to 8:30 P.M. This move will force Arrested Development to end in late April, which trims its number to 18 episodes. This does not bode well for the critically acclaimed comedy, which has been averaging 6 million viewers a week."
Link