I'm just pointing this out because I see a lot of griping about waiting on quest mobs and such. I was on belsavis, harbinger, the other night and roughly 100 people were on the planet. I switched instances to belsavis 2 and there were 4.
So it's just a guess mind you, but I'm guessing people don't know about it, have forgotten, or some combo thereof.
That being said, if you pull up your map, on the lower right corner youll see a dropdown menu if there is currently more than one instance available.