One reason that some people prefer the mono versions, is that at that time (1960s) a lot more care generally went into the mono versions. It was mono that got played on the radio, so it was considered the “important” mix that absolutely had to be done right. Stereo was a bit of a novelty, and stereo mixes were sometimes done quickly and only to satisfy the few people who had stereo phonograph players. As a result, some of the stereo mixes were lackluster. Also, as mentioned before, some of them were severely panned (including some of the early Beatles stereo mixes).

That said, there are some examples of early stereo that are fantastic, for example some of the Mercury “Living Presence” classical recordings.