How is the Socialist Millionaire Protocol different from a simple call-response such as CHAP?

This is a question of doubt, but what does the protocol of the socialist millionaire, which is usually used in conjunction with OTR-Messaging, do that the "usual" mechanisms for responding to calls, such as CHAP, can not do?

http://en.wikipedia.org/wiki/Socialist_millionaire

http://en.wikipedia.org/wiki/Challenge-handshake_authentication_protocol

PS: Can someone consider adding an "OTR" tag?