I'm sure interest revenue plays a role in determining the rewards that are offered, but the data suggest that merchant fees, which are largely paid for by non-credit-card-using households in the form of higher prices, are a bigger contributor. The average credit-card-using household receives a net transfer of over $1,000/year from non-credit-card-using households [0] (2010).

[0] https://www.bostonfed.org/publications/public-policy-discuss...