Because services as far as Banbury were under the remit of Network SouthEast, who introduced DOO as part of the Total Route Modernisation programme of 1989-1991. Services north of Banbury were operated on behalf of Regional Railways, so the DOO agreement did not cover it.
Implementing DOO would have been possible later on, but Networker Turbo operated services north of Banbury only started around the time that privatisation was beginning to happen (with the Train Operating Units etc.), so it was the worst possible moment to engage in an industrial dispute with guards. They couldn't sack all the guards either, because none of the other lines or stock in the area were DOO capable.
Implementing DOO would have been possible immediately following privatisation too (C2C did that as part of their fleet renewal) but it is clearly a battle that it was not worth Chiltern's while to get involved with.