Many mono tapes probably were dumped as it was generally thought that stereo was superior etc., and the companies wanted to save storage space. Not really logical, as most households still had only mono playback systems until the late sixties or even early seventies.
On the occasion of the SACD reissue of "Hohn Coltrane and Johnny Hartman" Rudy Van Gelder wrote that until that time engineers were instructed to take the most care in mixing to mono - the SACD includes both mixes to proove it, the mono mix sounds better. In view of that it was a rather stupid decision to throw away the mono tapes.
The only LPs I ever regretted selling are some original Atlantic mono pressings. Those I still have sound great when played back with a mono cartridge.