The ps4 pro is a costed design, they didn't spend too much money on the overhaul to get 4k. The xbox one X is basically not a costed design and will cost a lot more to make so while the xbox one x is a more powerful console it will really struggle to get close to the ps4 pro pricing. So I wouldn't be surprised to see a ratio of 10:1 of ps4 pros for every single xbox one x sold.
But really its down to marketing Sony hasn't really pushed the pro its been more ps4 marketing in general where as the xbox one X marketing I reckon will be better but if you do that then the standard xbox one will suffer and fallback even faster compared to the standard ps4.
I'm more than happy with my xbox one for now though and I won't be tempted to go for the X model I think for a long time even though I have a 4k tv. I use my xbox one a lot but it plays second fiddle to the ps4.