When I was @ JW HQ we discussed this in detail with some of the old-timers.
Basically, everyone is on the money:
1. Usually these guys have such strong influence that they could potentially take a large group with them if they join a site like this.
2. Also, if they get into trouble (which has happened in the past) like, adultery or something, the fallout can be especially hard and widespread.
3. Since, they are somewhat human, WTS doesn't want them to get too close to the brothers. They need to maintain some distance to allow better representation of the WTS edicts. Just imagine, telling someone not to go to school, when you know their children and family are suffering as a direct result.
4. You don't want these key men and women to be influenced by the 'real life' that the brothers are going through. That's actually one big advantage of bethel. Bethelites are so far removed from reality, their counsel is always closer to WTS direction.
Just a few things I've learned.