Fink's Television Engineering Handbook lists the following flicker threshold luminance vs. flicker frequency (for large area flicker):

Movies 48 Hz, 20 ft-lamberts;
TV, 50 Hz, 29 ft-lamberts
TV, 60 Hz, 180 ft-lamberts

Also stated: A positive increase in flicker frequency of 12.6 Hz raises the luminance threshold ten times.

So, you can see that the visibility of flicker is strongly dependent on image brightness.

It also states that the threshold is strongly dependent on the size of the flickering area, the surround brightness, and etc.

I can see now why they were compelled to do that in those days

I found that I needed to set my CRT based 17 inch VGA monitor to 85Hz to avoid flicker. It's only moderately bright, but only 40cm from my nose. That would relate to a large area as seen by my eye.

The last post makes the point well. The peripheral vision of eye (rods) is MUCH more sensitive to large area flicker than the central zone (cones). You sit much close to a computer monitor than a TV so it subtends a much larger angle. Hence flicker is much more noticeable. That's why 60Hz on a TV is usually OK but is truly horrible on a computer monitor.

You can argue that 50Hz and 60z are not OK for HDTV because they subtend a larger angle than SDTV when used at the intended viewing distance. LCD displays inherently have no large area flicker. Other types such as plasma might well benefit from up conversion to 100/120Hz.

IIRC, I think Cuba was an early adopter of NTSC TV in 1957, IIRC. Havana had a color station before Miami did.