Movie/film makers had great quality cameras/equipment by the late 1930's - so I guess it is a combination of the movie/film being available/remastered in (say) blu ray quality (as opposed to VHS
) and - as you say - having a TV which is capable of utilising that blu ray/HD quality.
We rewatch many older films if we think they have been remastered/transferred to blu ray - so nice to be able to pick out details (except perhaps the aforementioned June Allyson night attire
)