Better question - why not?
Because crypto payments are niche and likely come with more hassle (e.g. volatile value, lender needs dollars and has to take on any risk with liquidating bitcoin/other cryptocurrencies). You might as well ask why US mortgage lenders don't accept payments in RMB or Euros.
Exactly.
>> It's an additional source of revenue for them.
Not all revenue is good though. If I spend $1,000,000 to get $5,000 in extra revenue, it would be a pretty bad business decision. A lot of crypto is hype, and I'm not sure there's much actual real revenue in supporting it for anything besides speculation.