Because the likelyhood of it happening again if he sat down with Shearer and worked out a way to work with him would be minimal. Shearer would give the whole place a massive lift and give our chances of promotion a huge boost, thus Ashley would be able to sell the club for more in the long run. I also think if Shearer wouldn't be as ready to leave if things went to shit as KK was. Shearer knows we're a shambles at the moment so won't drop everything and run at the first sign of difficulty.
I think while he's still trying to sell the club he didn't want to appoint Shearer because he might not be the preferred choice of a potential buyer but if he does take it off the market, bringing someone like O'Leary in ahead of Shearer will be absolutely beyond belief.