The 60's did not have the technology we have today regarding weather. There is no excuse from a pilot perspective regarding weather unless it is in controlled airspace whereas that responsibility falls on the ATC tower since pilots are typically "head down" on land preps.
The pilot had one of two things on his mind;
"Fly through it with risk, land and get to the hotel and bang this flight attendant that dropped Viagra into my coffee"
or
"The company emphasis's using as little fuel as possible, I'll take the risk and make my log book fuel-usage look stellear"
Except for the southern Airways accident happened in '77 and many of the planes from that era where still flying until just a few years ago.
Airliners, in the US at least, are always in controlled airspace, except when arriving/departing from the very few untowered airports that see commercial service. Except for the visual approach, almost every commercial flight is conducted under instrument flight rules.
Modern Radar in aircraft can't even detect hail and as it has already been pointed out, hail can be experienced up to 20nm from the cell. When you have a line of storms hundred miles across, it's either take the best route possible through the cells with the available information given or terminate the flight. Encountering hail like this is rare.
Someone brought up Delta's 747 hail incident in china. That was because in China, the military owns the majority of the airspace and there are only small paths through the military airspace for commercial airliners.
The crew probably just shot an automated landing.