It's certainly not because it is a rarefied classification. There was a time when profession was reserved for more obviously learned occupations such as teaching and law. But in the past 50 years it has been extended to encompass those in business support functions including human resources and information technology, whose roles are certainly no more significant than those controlling occupational risk.
Perhaps keeping that distinction between OSH management and other disciplines is the point for many who would rather be known simply as practitioners. If they see their work as morally more important then that's understandable. Ensuring that profits and efficiency do not come at the price of workers' lives is vital work for society as a whole.
Practitioner is an honourable term to be sure and serves those in the medical world well. But its shortcomings as nomenclature become apparent once you go looking for a collective container. A professional belongs to a profession but what is a practitioner part of? A practice?
It's also self-limiting to define yourself by distinction. OSH managers who see themselves as set apart from other organisational support roles are less likely to pursue the kind of involvement with their colleagues that generates some of the most valuable ideas.
IOSH's Blueprint competency framework places a hefty emphasis on the broader business skills and organisational understanding that make rounded OSH professionals/practitioners. Those are more easily developed by engagement with peers in other disciplines.
Whatever irks some OSH specialists about the label professional, it is probably a generational phenomenon.
The day is passing when it will be possible to work in the field with only a certificate saying you have spent days learning the basic principles of OSH management.
In the next couple of decades tens of thousands of new OSH managers and consultants will be needed to replace those due to retire. After they have put in the years of vocational study needed to become competent in the eyes of an employer, it is unlikely those younger entrants will want to see themselves as anything other than professionals.