For the elite guys, the answer is simple... money.

50 years ago, Floyd Mayweather would have fought 7 or 8 times a year, because he'd need to fight that often in order to make a living. Nowadays, he can afford to fight 2 times a year, because he can make 20 million in one night.

The old school fighters would defend their titles only 2 or 3 times a year, just like today's champs, but the old guys would also fight several non-title bouts in between to pick up extra cash.
If the big paydays were available back then, guys like Robinson, Armstrong and Pep would have only fought once or twice a year.