I would agree that it's who you know (and more importantly, how well they know you). It's a liability issue. The captain of the boat is responsible for your safety. If he doesn't know you (or your dive experience), why would he just let you take off in the open ocean? He's paying that dive guide to take care of you while you're underwater. So, if you want the freedom, then you have to put in the time and earn it.
By the way, Ocean Concepts does not own their own boat. Although Ocean Concepts only books divers on that one boat, the boat itself is no longer Ocean Concepts exclusive. The story around the island is that Ocean Concepts doesn't have enough business to occupy enough seats to make the boat profitable. So the owner opened it up to other shops. There is another boat operator on the west side. Captain Bruce (name of the company, not the actual captain) also dives the west side daily and has a nicer boat and a MUCH nicer staff. Aaron's will dive west side..but not daily.