Just to add to the very well done explanation by Rongway, some of it comes down to the simple fact that almost any content that is not current isn't worth running sync'd. The gear is for glam only and the difficulty of doing it sync'd is still there. The content can be easily cleared with an unsync'd party which you can't do within Duty Finder. So no one queues for it.

I'm not saying there isn't value in running content for the difficultly. Finding 7 other likeminded people might be hard. So, assume anything extreme/savage/higher is party finder only.