Money. American Express merchant fees are higher than Mastercard or VISA (though not nearly as high as some on Flyertalk think) and this means Amex can give the issuing bank a higher percentage.

More Money. American Express won a large anti-trust judgement against the owners of Mastercard and VISA for illegally blocking American Express from issuing co-branded bank cards. Guess who the owners of Mastercard and VISA were? American Express then offered to waive the judgement amount for those banks which agreed to issue American Express network cards. Over time this will erode the acceptance advantage that Mastercard and VISA created.