Delta Force was created in the 70's and was and is run in the same way as the SAS in that it has approx 5 squadrons with each having a small group of men with their own speciality.
The guy who created Delta force presented his recommendations to the Pentagon after spending time with the SAS...
Ask yourself these....
1. How many paras have joined the SBS
2. How many marines have joined the SBS/SAS/Mountain Leadership cadre
3. In arctic/mountainous terrain, the RM/commando are specialists
4. The paras are a specialist/airbourne front line infantry, the RM are frontline specialist...