Personally, I think the division of services harks back to our imperial class system. Aerodio were ex-enlisted men, while ATC were drawn from the officer ranks. ATC kept the airliners apart, and FS looked after the rest. Keeping aircraft apart was Officers work, and passing weather and grubby HF work wasn't. That division persisted, even though the underlying educational qualifications became irrelevant. There was definately a supposition, particularly amongst ATC, that FSOs were either failed, or unsuccessful ATC applicants. I didn't know anyone during the time I was an FSO who was either.