For the most part it was a need to fighters back then. The need to survive and earn a living for their families.
Of course there were other means but who the hell wanted to break their backs and work long hours for chump change?
If you notice a handful of fighters from back in the days were somehow involved with ilegal types of money making schemes.
I'm not saying all but a lot were, I'm certain theres more we never found out about.
From throwing away fights, to being involved with the mob to drugs.

The easiest way to make some cash was for the most part the best at that time. Even today fighters who come from the streets and are hungry tend to be hungrier. You do have some fighters that come from good families and are well educated who have maded but for the most part is the fighters that have gone thru hardships that use it as there fire and fuel to succeed.

I think it's a mixture of a lot things that were happening in those eras the economy, life it'self, promoters, health etc. etc. etc.
All of those things added to how much a fighter fought and how badly he had to win in order to survive.