The difference is that I trust Amazon far more than those other companies you mentioned. If they go out if business or even change their "unlimited" policy, you're exposed until you can get your 2TB re-uploaded to another provider. It's a pain and a risk I'm unwilling to take. I know Amazon isn't going to suddenly try to dump me as a customer.
Still, it's easy to improve reliability by introducing redundancy by using two backup services. And a total of $9-15/mo is still cheaper than $20+/mo. I presume the chances of multiple independent companies turning back on me at the same time as reasonably low. Well, it's not either completely impossible that even Amazon may have some hiccups, too.
