That's easier. The Keynesian origin of AS/AD (though much has been written on the topic recently by several readers of this list) makes it unlikely that prices - sticky as they are - may play the role of the model's adjustment engine in case of disequilibrium. The standard textbook's explanation of the two curves' slopes mixes up classical and neoclassical mechanisms (say, Pigou effect or Lucas's islands model) in a basically Keynesian framework (i.e. when classical dichotomy doesn't hold).
Maybe you should look at Irving Fisher's macro model for a more consistent explanation. 
Nicola Giocoli
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