There are several reasons why the merchant would reject a publisher. It might be that your promotional site does not fit in with the merchant's brand image or the merchant does not work with certain types of publishers, i.e. coupons or incentive sites.
The reason why you have been rejected is generally indicated in the notification email sent to you following a rejection. In case you haven't received a reason, or would like to negotiate further, please feel free to contact the merchant directly as they are responsible for validating their publishers