The problem with online payments is chargebacks. Except now instead of say a few hundred dollars, what happens if you have a tenant that does a chargeback for half a year's worth of rent? That's a LOT of money!!
And unlike say a retail company where you can vet your own customers, you cannot vet the tenants of your customers (the property owners). Therefore if someone is renting to lower quality tenants, and there are chargebacks, what happens? Who's responsible? How long will it be before you start to have problems with the credit companies? How long before the payment processing company will require all kinds of hoops before they accept people into their systems?
You basically need to become a Paypal for online payments. And so most of your time ends up being spent dealing with fraud because the amounts are so big and as a result the impact is so large. That's the hard problem you have to resolve to stay in that business.
All the while remember that payment processing is a low margin business. You cannot charge more than 3-5% for accepting credit cards. For many property owners paying an extra 3-5% to accept rents is a non-starter. remember that real estate renting is often a business built on leverage, so when you make 5%, that's 5% of the leveraged amount and can be quite significant in comparison to your cash investment. It's a double edged sword, and getting hit with an addition 3-5% can have a significant impact.
During all this time remember that there are tenants who are actively looking for these kinds of rentals, to take advantage of this possibility. They aren't the majority, but at the same time the majority of rentals don't accept credit cards for rent payments.
What we tend to see people asking for this are trying it for the first time. Most will eventually find a way to try it, and after a short while the vast majority stop because of the above. You generally only see the bigger companies offer it over time, and that's because they are bigger and can more easily absorb the issues. Not only that but they have processes in place, have better negotiating power, and so on, so it's easier in comparison for them. But that generally means you're dealing with companies with thousands of units, and they aren't the bulk of investors and/or property management companies in the market.
It's a tough business. It's really a subset of a Paypal/Stripe type of business, but with larger amounts and biggest potentials for fraud...