Naval aviator & NFO attire while underway

Just started reading Sherwood's new book "Afterburner," about naval
aviation during the Vietnam War. While examining ready room photos of
various naval aviators and NFOs, I noticed many of them are *not* wearing
flight suits. Instead, they're wearing trousers, a web belt and a long
sleeve shirt that buttons up the front. (Sometimes this garb seems stiffly
starched. Other times it's sweat-stained and quite wrinkled.) The same
holds true for guys depicted in photos found in other books, such as the
two-volume series on MiG killers.

By way of contrast, whenever I see current day naval aviators and NFOs in
various TV documentaries shot while underway they are *invariably* wearing
flight suits in the ready room, dirty shirt wardroom, etc. Can anybody
explain the change in underway attire between the Vietnam era and today?
Is this controlled by a regulation? By the skipper? By some unwritten
tradition?