when you find deleted scenes sometimes it's a f.. miracle with it's a mad mad mad mad World (1963) in (1989) or (1990) they found some while about to tear down a warehouse that hadn't been used in years, so it was just by chance. for Friday the 13th part 2 look up the recently recovered footage from a couple years ago they found really by accident the uncut gore where the MPAA had forced them to trip the gore in part 2 to get an R rating. that was thought lost until by chance the make up dude that worked on it just by chance found a trunk of his stuff from FF13 part 2 (1981) and a VHS tape that he had no idea even still existed and he hadn't seen the footage since working on the film in (1980) no sound but you have more gore. sometimes you get lucky like with The WIzard of Oz (1939) where you have some deleted footage but what studios used to do is just pitch it they'd throw it away.
in the 30's for example sometimes MGM would film an entire film it'd be finished and they'd show a preview of it to an audience it test badly cause they don't like the lead for the role whatever so they'd recast it and start filming with an entirely different cast and than put the old version in the vault and not one of those has ever surfaced as far as i'm aware. one i'm dying to see but i doubt it's still even around is (1983) Original Version of Smokey and the bandit part 3 where Jackie Gleason is in the role of the bandit and the sheriff. it confused audiences to they hired Jerry Reed to play the bandit. on you tube there's a teaser for the Unreleased version that's all that has ever surfaced. Deleted Scenes these days are kept and than you have what ends up happening to all the footage that was shot for Night of the living Dead (1968) leaving out the film but the outtakes the deleted scenes, bloopers everything if i recall years ago was in a flood so that footage was all gone cause of that.
Fires in studios cause the film it was on a certain kind of film that had to be at a certain temp and if it wasn't it would catch fire and that's how we lost so many films. that's why Orson Wells has so much lost he had a fire in his house in (1967) and i think he had another one in (1970) if i recall. it's just utter luck that Dallas has any footage still around i think. it's even more luck that Dark Shadows has only 1 episode entirely missing because tv networks would tape over the episode from the day before. which is why a lot of shit was lost this happen during the BBC shows in the 50's and 60's. they'd junk them you can find certain episodes for certain soaps but never the full year or run of the show cause it was just luck if you found one. hell look at The doctor who stories that have been lost a lot of those have been found just by accident. others on purpose i think the deleted scenes of the film Metropolis (1927) that were found what 10 years ago? was found in a basemen of a church that i think was abandoned. so sometimes it's just luck