Anecdotal, but every small business owner I've talked to have said it's because of the 1% difference in fees, even with OptBlue.

I've only had 1 business owner ever say it was due to chargebacks, a small tea shop in a Chinatown in Canada. They also don't take Mastercard for the same reason.

This is also why American Express launched the Cobalt card in Canada, which had a 5x multiplier for eats, drinks and groceries, and pushed it so hard for years on social media and bus shelters, despite eventually starting to disincentivize it (it was a loss leader/upgrade feeder product for Amex). They wanted to win merchants back with a new generation of hip, social media savvy Amex customers.