Apparently, he takes too long to warm up to be an option in the bullpen. So, it was either part of the rotation, or left off the roster.
They talked about it last night on TBS. Bochy told him they were leaving him off the playoff roster and Zito apologized for faltering down the stretch and even having to put Bochy in that situation.
Of course, I'd love to have Zito on the Nats. I'd take a 4.15 ERA and 1.34 WHIP over a ton of the other stiffs we drag out there on a nightly basis.