Pro to X isn't exactly an apples to apples.
First of all, the Pro announcement/launch was much more low key than the X. It was advertised as an unnecessary item, that was mainly a nice upgrade for the more avid gamer, while the X was basically marketed as a new generation in gaming. Second, the Pro was launched just a few weeks after its announcement, while the X had more than an entire year to build up hype, and 3 months worth of preorders. Third, many felt they didn't need to jump to the Pro, as they were still getting 1080p in most of their games. Many XBO gamers were tired of settling for 720p-900p on a lot of their games, so the jump to X was highly sought.
Finally, and most importantly, Sony completely underestimated the demand for the Pro. It was in and out of stock for the last bit of '16 and the beginning of '17. MS had quite a bit more time to gauge demand and has not had the same stock issues that the Pro did.
