There definitely was recorded incidence of this - Washington Square Park for example - but the main reason for the difference today was due to vagrancy and loitering laws, as well as 1930s Dust Bowl governance, that no longer exist for human-rights reasons.

Very easy to keep the parks 'beautiful' when you could just shift anyone undesirable to a prison, asylum, or skid row.

N.B. Littering also wasn't seen as a social failing in 50s America, so there's a bit of a juxtaposition there versus the contemporary reality.