The fine was the maximum OSHA could charge DuPont, a many-multi-billion-dollar company for multiple counts of negligence that led to the death of four workers.
Because, of course, Congress couldn't possibly beef up penalties for lethal safety violations. It might cause some job-makers to not make as many jobs for people to die in.
Paying fines less than the cost of preventing the issue is just good business sense not a punishment.