Because Motorola isn't selling them a license. Ah, so why isn't Motorola selling them a license then? Because Apple is unique among the big mobile phone players in that they have no interest in licensing their own patents to others for money or for access to the other party's patents. Instead Apple are using them just for the purpose of getting the others off the market.

Nokia apparently managed to bludgeon a cross-licensing agreement from Apple. Motorola is presumably attempting the same.