The current FC rank and conditions of service were laid down in a DCI that was published around 1990. I think I still have a copy but the SNCO rank was partly to encourage people to the job partly to reward them and also partly to give responsibilty a rank. Also the punishments for SNCO were/are far greater than those given to Airmen or JNCO's.
One of the reasons was that the FC has to deligate part of the job to an Assistant so making the FC a higher rank allowed them to give orders and not requests.
But then again I've been out for a number of years now and its probably changed