Let me write something here as I speak from experience.
Some credit card processors actually demand that there's no other payment options on your site, so that they can offer you a better deal.
Others are not as 'cruel' but still offer something similar.
Take paypal for example: Normal paypal fees are 3.5% over the total amount, but if you as a merchant have more income than $50.000 per month through Paypal, that fee goes down to 3.0%, giving you 0.5% profit, which is quite a bit. I don't know the profit margins of onlyfans, but hypothetically let's say for every $ you spend, 70% goes to the model, 20% goes to the onlyfans staff, 5% goes to hardware and facility, that leaves only 5% left. If Paypal takes 3,5%, that leaves 1,5% profit. But if Paypal takes just 3.0%, profit goes up to 2%
That's a 33% profit increase!
Let's, for the sake of argument say that a second card processor has a similar arrangement, and onlyfans has an income of $75.000 per month.
If they just had Paypal, then the money going through paypal is $75k, so they get the discount.
If they used 2 payment methods, they'd have ~37500 going through Paypal and the other through another, and none would have the discount.
It's all a bit technical, but that's likely the reason for them to stay with just one payment provider.
That, or their staff isn't competent enough to embed other payment methods