Respirators are uncomfortable to wear. I know that might seem like silly reason considering the long term health consequences, but in industrial settings it's quite common for workers to ignore or disable any safety equipment that gets in their way.
You've just hit the nail on the head. PPE is an annoyance, especially in the form of devices that make critical bodily functions (like breathing and dissipating heat) difficult. Regulate all you want, workers will voluntarily remove the PPE often enough to incur respiratory damage just like they do with hearing protection in areas that aren't immediately deafeningly loud.
I listened to this on NPR earlier, didn't reread it, so I don't know if this isn't present in the piece, or other posters just didn't read. Miners said that the respirators got clogged quickly, and it would feel like someone had their hands over your mouth. And the respirators that are recommended and provided protect you from coal dust, but not what causes this disease, silica dust.
Not really sure what the solution is then. And it's easy to see why companies wouldn't bothering doing much about it.
The solution, as other companies have found, is for management and unions to work together to make safety a core part of the corporate culture.
Do you own a respirator? Why not?
I actually do, for wood working. Won't be breathing in all that sawdust.