It's just a general difference in how beast tribes work between ARR and Heavensward. In ARR you get a new quest giver with each rank and can choose to focus on one tribe with most of your allowances. In HW you have one quest giver with new quests added to their pool when you rank up and are limited to 3 per day (except on days where you hit a new rank and can do 3 bonus quests).
We can't know for sure why SE made this change but I imagine they wanted to limit how fast you can rank up with each tribe.