For me, the reasons are that I love my current Ipad pro, I don't have a good reason to upgrade it, and the pricing isn't advantageous.
I change my Macbook often because of work-related reasons (I.e. when changing job I need to get a new one)
I change my iphone often because the new ones have a big enough differentiator (for me at least), but more importantly, the way the pricing is structured with my cellphone provider, I almost get it for free. (I.e. I had to upgrade my plan to get more gig/months and a few other features so I only had to pay $200 for the new iphone 12).
But for the iPad pro? I'd need to pay the full $1500 upfront and the new one doesn't offer a big enough value add compared to the previous ones.