I’ll give this a shot: NFPA sets the “national standard” for fire operations and equipment thru a consensus view. Not everyone agrees 100% but the majority forms the basis of their standards.

Then all across America, the NFPA standard is published and in some places it is adopted. Everyone is free to choose what of the many standards they will adopt follow or ignore. The problem is that if you choose to ignore the “standard” and bad things happen, you are usually liable in a lawsuit. (My soapbox comment: Bad things happen, people die and somebody is usually to blame for it and those folks need to be held accountable for defective equipment or practices that are not safe.) Apparatus manufactures and others including cities and states do not want to expose themselves to additional financial risk needlessly so they adopt the NFPA standards they feel provides them the best protection from lawsuits. Maybe that is why they are moving away from commercial frames.

I hope that answers your question.