Through 116 voters, the core rulebooks average 111 each (96%), the three pure rules supplements 88 each (76%) the settings 54 each (47%), and adventures 50 each (43%). I'm not counting AI, Theros, the box sets and Tyranny.
What we don't know is how well that 76% rate for supplements would hold up with more books published. It would probably dip, but how much and at what point? And then how to balance that with work required and the danger of bloat? Presumably WotC thinks one every two years is the sweet-spot. Doesn't mean it won't change, but it feels quite deliberate.