Which titles are pan & scan only? Why

Some titles are available only in pan & scan because there was no letterbox or anamorphic transfer made from film. (See 3.5 for more info on pan & scan and anamorphic formats.) Since transfers cost $50,000 to $100,000, studios may not think a new transfer is justified. In some cases the original film or rights to it are no longer available for a new transfer. In the case of old movies, they were shot full frame in the 1.37 "academy" aspect ratio so no widescreen version can be created. Video shot with TV cameras, such as music concerts, is already in 4:3 format.

There is a list of pan & scan titles in the Film Vault at DVD Review and in the Internet Movie Database (which also includes discs with both widescreen and pan & scan versions).