Part of the reason they probably went with level 60 instead of level 50 is that, at least for players who have been at the game for awhile, there's no guarantee they've actually played a level 50 job recently. Before I picked up DRG a month or so to finally take it to 60, I hadn't touched it at all since 2.2, which was in...March of 2014, I think? That was two years ago—if DRG had been my only DPS class, I'd probably be pretty rusty on DPS fundamentals by that point. Anyone with level 60 on a role, though, has likely played that role sometime within the last few months, which means they'll probably be more comfortable offering pointers and the like. This isn't to say that someone who hasn't tanked for 2 years couldn't offer tips—but this is just one way to make sure that the mentors with those roles have recently engaged with them.

It's also probably level 60 in part to "wow" new players. Having someone who has obviously invested a lot of time into the game take time out to help you will probably feel cool to new folks, and level 60 is, ultimately, more impressive than level 50, since a newcomer to the game may not realize that the old level cap was 50.