Bitcoin is not being used by big companies for the same reason why BitPay is popular.

Too Long; Didn't Read

The reason is quite simple: you cannot enter a legal agreement with Bitcoin. Bitcoin is above the law. It exists in reality where things are not depending on the law. You can call that reality a code, IT or simply mathematics. No matter what you call it, it’s based on science and science obeys only it’s own rules. Limiting Bitcoin with law is as stupid as limiting gravity with the law. Yes you can punish people who use Bitcoin, but it will have no effect on Bitcoin itself. Likewise if you punish people who obey gravity it will not remove the gravity itself.