Trainers didn't really want to train them citing lack of coordination and finesse required of a fighter, and the big man style wasn't that well developed yet (I believe.) Add to that the different refereeing (less hugging, more infighting etc.) and you get a rather mediocre environment for the tall style common among modern heavies to thrive and develop.
It's a bit of a paradox on here, the posters on Classic believe that boxing has regressed and that fighters/trainers are generally poorer than days of old. The question is then why weren't this old giants of the past as good as the HW's today, surely that should be the case if they were under the tutelage of better coaches?
"Why were runners in 1850 not as good as runners now? Why could they simply not train them as well?" Why could pitchers in 2018 throw 10x as many 100mph fastballs as pitchers in 2008 could? There are gazillions of reasons for why athletes get better. Although, in that time, people in general were shorter, tall countries in Eastern Europe didn't compete and Africans weren't allowed to compete for the title for much of history, cutting the competition even lower.
As stated, for one, there are just more big guys now, so you're more likely to get good ones. For another, I wonder if it has something to do with how big boxing was back then. That's where the money was. So maybe you had tall athletic guys who were maybe better suited to other sports nonetheless go after boxing because they wanted to earn. Today, basketball is a far better route for a tall athletic guy if he has the talent. No sane big man more suited to basketball would touch boxing with a ten foot pole, even if it is fun to imagine a young Shaq having taken up boxing.