The old 1/144 scale Otaki kit has been around since the early '80s (maybe even the late '70s?) and was repackaged by both Testor and Revell. It's not clear whatever happened to the molds for the kit. Supposedly Revell had them but they may never have actually obtained them. Nobody who knows has said. There's a possibility that the molds were lost in a train wreck/ship sinking/zombie apocolypse -- don't know. Existing copies have been going for lotsa bucks on vendor tables or on-line auctions.
But that may change soon. Roden has announced that it will be producing a NEW 1/144 C-5 Galaxy perhaps later this year. They've recently done a 1/144 C-141 (reviewed recently in FSM), so this is a logical follow-on. Just wish they would do it in 1/72 scale, too!