It's required in class A, B, C, and E above 10,000' MSL. And within 30 nm of a Class B airport (mode C veil). That leaves a ton of controlled airspace in which

It's required in class A, B, C, and E above 10,000' MSL. And within 30 nm of a Class B airport (mode C veil). That leaves a ton of controlled airspace in which it's not required. There are more class D airports and airports with class E surface areas, than there are class B and C airports.