I know a lot of people have had big issues with AmEx's automatic chargeback policy when a customer reports a fraudulent charge. Basically, people have been abusing the crap out of this system, and for merchants to get their money back they have to confront each client in court. Obviously, it's much easier to drop them, which a vast number of merchants have.
Also, AmEx's transaction fee's have tended to be far higher than other credit companies, especially during this credit fiasco.
And as always, there are a lot of things that go on behind the scenes with credit companies. I'm sure SDC has a very good reason to drop AmEx, it obviously isn't easy for them.