I believe it’s mostly because of the payment trouble you have to go through.
Not that there are no workarounds, but I am guessing affiliates are getting tired of running a business like fugitives.
Since the US government seems to have a direct impact on the whole situation, I believe this is what suits them best, FOR NOW.
The real question then would be Why would the government want sports betting online in the US to feel like a teenager having sex without a condom? For the money? Nah they can’t be that greedy *sarcasm*