Some of it is likely due to changes in the code of points - the amanar has decreased in value relative to the DTY over the years (it's still worth more, but the jump in D-value from the DTY to the Amamar decreased for the 2016 quad and again for this one). For the 2012 quad it was worth a whopping .7 more than a DTY, and for the 2016 quad it was worth .5 more. No wonder everyone who could possibly manage an amanar in 2012 was attempting them in competition at least some of the time. But it's only a .4 difference now, so gymnasts can often make that up (or make most of it up) performing a clean, consistent DTY over a "maybe she'll land it, maybe both her knees will explode" amanar. I think If they're not particularly strong vaulters, they may be able to better use their reps and energy upgrading elsewhere now.
And of course there's just somewhat of an ebb and flow and cycle to which events are particularly "stacked" in the top-performing countries and where gymnasts are pushing difficulty (ex. some years we're desperately looking under the stands for strong bars routines while we ooze floor phenoms, and now we've got plenty of gymnasts able to medal on bars but floor is weaker than last quad).