It just seems to me (do not have my jug to 50 yet) that jugs are just way better at tanking than sins. Yes sins have stealth and do better damage (at least single target) than jugs when both are wearing dps gear but the better teams for ranked usually want a good tank jug. Not full tank but a hybrid, at least up to unstoppable in vigilance.
I explained why rwzs use guardians as MTs. It's because of friendly leap and free spammable aoe slow. These two things make them a better tank then the other two. It doesn't really have anything to do with their def cds because a sin that is darkness/deception hybrid will have 24s of 25% dam reduction through blackout along with shroud and deflection. That's pretty equal to guardian cds, esp since blackout has a 1 min cd.