Generally a crime needs mens rea. Not always but generally there needs to be some sort of intent. It is unclear from the article if the violation of the safety practices were intentional or due to ignorance or potentially lack of training on the employer's part. Since the article does mention that policies and procedures have been updated to account for this there may been some deficiencies discovered in their policies.

This is usually sufficient to explain why criminal charges have not been brought. If it was just simple incompetence but they believed they were following procedures to the best of their current knowledge it is unlikely they could get a conviction.