Well, if you think about it, let's say they do all of Shurima. Azir, Nasus, Renekton, Xerath.
They have to include Taliyah who has her own story.
Taliyah might come across Rammus or Amumu. Or better yet
There is Sivir who helped bring back Azir and Cass who stabbed her. That means they will have to work on all of them simultaneously. And before wego into "Well Rito is working on that" or whatever reasoning akin to that, the problem is everyone has a story. And while they can do a follow up, if they wrap up a champ's story within 1-2 months after they get their intro, then any lore-nut who is a fan of that champion will be sad because their champ will not show up again in any meaningful way.
I mean, Azir COULD pop up randomly, but then, his main story is already finished, there is no other true quest for him to accomplish as of right now, and Riot wants their champions to have a reason for meeting the others. Not because of "Rule of Cool" but for proper story building elements.
I rather see Azir's story come out over the procession of a year to a year and a half (I mean, if it's that long) as opposed to getting all of his development in the span of 1 or 2 months.