Asciano Sforza's got a good head on his shoulders. What incentive would Rovere have to follow through on restoring Milan-Napoli relations once he becomes Pope? Better to turn him down now and work on building a foreign policy presuming that Naples will remain an adversary.
As disgusting as it may be, a marriage alliance with Borgia's daughter is a much stronger bond to cement a working relationship. Poor Lucrezia though. Women really are treated as nothing more than political pawns.