Some content can't be completed solo or even with two players even if the ilvl were high enough due to the game mechanics involved. Even some content that allows you to go undersized has restrictions where you can't solo or duo without glitching it just right. But this isn't the main reason, the main reason is probably because they don't want to confuse people by making them think they can complete content that requires much better gear to solo / duo it and perhaps want people to fill up the duty finder. Eventually once the ilvl goes up, maybe to 580+, the level 70 content will be unsync. This will be some time from now, like 2 years away or so, probably.