Echoing what some of the other members have said, it boils down to exclusivity contracts.
The top pros are under contract with outfits like MD & FLEX. Because of which, their contracts state that they are not able to post in any other forum, but their company's forum. The supposed reason for this is that an athlete under contract essentially represents that company by way of his actions and reputation, both in person and online. As a result, these companies want their athletes to post solely within their forums and forbid them to do so anywhere else, sometimes even under penalties.
It has NOTHING to do with Ron, as Ron actually makes it very welcoming for pros to post here (Gives them their due recognition, some privileges within the forum and also resolves direct conflicts they might have in definitive fashion). This is a GREAT forum for pros to air out their frustrations, as has been seen in the past (examples: Milos, Priest, Shawn, Prince etc.) and also a good place for them to break out of their perpetual "trainer" mode, as in they don't need to constantly hand out advice and promote certain product lines or websites by way of contractual obligations.
All that said and done, money rules everything. If you are an IFBB pro and have a wife and kids and are offered an extra 30-50K/year (That's what these contracts are truly going for) to be with a company that will ask you to fly out to different places throughout the year, drop a few comments in a forum and take a few pictures drinking a certain brand of protein or some other supplement, YOU will typically drop everything else and join that company for the betterment of YOUR family.
Joining that "company" will also keep you relevant in the bodybuilding industry, which is important due to the relatively short shelf-life that most competitors have unless they win a few pro shows or garner a significant following via other "venues".
Again, Ron is actually seen as a fresh breath of air in this industry and is very well-liked due to his excessively diplomatic stance towards everything. Ron respects EVERYONE in this industry. He never says a negative word about any company or individual and he always does his absolute best to protect the IFBB, NPC and the industry as a whole.
"1"
P.S. What I will say is that MANY IFBB pros post here, but do so under the guise of random usernames.