John Gruber wrote a loose theory on the continuing cooling of iPad sales.

The peak years (2013 and 2014) were inflated because it was an untapped market. Steve Jobs was right, there was room for a new device in between a phone and a laptop, and the iPad was and remains an excellent product in that space. But people don’t need to keep buying new iPads. I think the replacement cycle is clearly much more like that of laptops than that of phones. This was not obvious to me at the time, but it seems obvious now.

People use iPhones and iPads differently and the replacement cycle is only starting to emerge after being confounded by the demand after it was released.