Why does Philadelphia leave off the mint mark on pennies?

Because prior to the 1980s, and a few exceptions (such as the "war nickels" of 1942-1945 and the Susan B Anthony dollar) since the Philadelphia mint was the main mint, it didn't use mintmarks. When the P mintmark was added to the quarter, dime, nickel and half-dollar, it simply was decided not to use the "P" mintmark on the cent.