Complex sentence: Traditionally only NCOs from the Army, Royal Marines and Royal Air Force were eligible to apply, as unlike members of the other services, members of the Royal Navy do not swear an oath of allegiance to the Crown.
Split into simpler sentences: Traditionally only NCOs from the Army, Royal Marines and Royal Air Force were eligible to apply. This was because unlike members of the other services, members of the Royal Navy do not swear an oath of allegiance to the Crown.