There is no reason not to and plenty of reason to do so. For example; why force previous expansion content on players?
By making them trad-able, (which is how they were in T5) people will then just do the Hunters Quest on multiple characters to earn the reward they want on their main. This leads to people getting done with their main sooner and stop running the Hunter's Quests. Don't get me wrong, I preferred that method. Some of us even made alts for the singular purpose to get Hunter Coins faster on our main. However, DBG would rather you log in and do 1 Hunter's Quest a day for two weeks; instead of doing 4 Hunter's Quest a day for 3-4 days.
Is Hunters Quest code for something else, or is it something I have just been missing this whole time?
Hunters Missions are added in TLE in each mid expansion update. It is a repeatable quest for hunter coins that can be turned in for BIS items. If you are just now hearing about them... Dunno man.
first hunter is in lavastorm, just go straight when leaving the tunnel. Second is near the shimmering citadel on naga island in Pillar of Flame. Required level is 50 for the first and 60 for the second.
Acronym meaning best In Slot, even if one could argue that the very notion of Best is vague, it means very good item and often the best you can hope to get outside hard core raiding.
BiS is best in slot, be it from raid or otherwise. In this case, some of the hunter rewards are simply the best thing you can put in this slot from any content available in game.