The sort-of-official reason to shut down the Mentor group is just unmanageability. A handful of Linden employees at the vTeam had to successfully communicate to a group of 3,500 rather independent volunteers, of which just a few dozens kept in touch. But even if all kept in touch, it would be absolutely impossible to make sure that each and every one strictly conforms to LL's mentoring guidelines.

The alternative, which is more manageable, is having smaller self-appointed "helping groups" of 30-50 people, each staffing their own "helping areas" (whether "official" Community Gateways, or just "unofficial" areas set up on private land), and having a representative giving feedback to LL's teams. Since these groups are independent and not coordinated by LL, they don't carry the responsibility for "representing Linden Lab" and thus can be coordinated much more loosely — and easily, since LL just needs to meet with the group coordinators.

But then again, it might just be an excuse, and the real reason can very well be that statistics have proven that even helping out newbies didn't really make them return to SL...