Because it wasn't available yet. The Apple III was released in 1980; the W65C816 wouldn't show up until 1983. Apple went on to use the '816 in the Apple IIgs,

Because it wasn't available yet. The Apple III was released in 1980; the W65C816 wouldn't show up until 1983. Apple went on to use the '816 in the Apple IIgs, though.