Dan,
SATCO and ATSM are fundementally the same thing, and although there may be airports that have both, I don't know of any.
I think it all comes down to the concept that SATCOs were managing ATS units anyway, and all around them, other supervisors in different airport departments were suddenly becoming "managers" and full of their own self importance, so why shouldn't SATCOs?
It probably started within NATS, and what's good for them is good for non NATS egos.
