I thought about this for a while, and my guess is simply "time".
Both teams have limited resources for developing their hardware. The PS4 Pro (and PSVR) were released about a year before the XBox One X, which means that the Playstation team could start full steam earlier.
The PS4Pro is very similar to the OG PS4 (doubling the amount CUs, add some features from the newer AMD chips), while the XBox One X is very different to the OG XBox (the ESRAM was dumped, going from DDR3 to GDDR, way more and more modern CUs, way higher clockspeeds then Sony with same manufacturing and a lot of other things I currently do not remember which are quite challenging. I expect that Playstation could put some of its teams to the PS5 development very early on, while Microsoft had to put a lot of resources to the XBox One X development.
This are just guesses of course, but they could explain that Playstation devs got their DevKits quite early. I think first rumours appeared in early 2018 (just a few months after XOX release), and we heard about problems with them getting too hot and too loud. But Playstation had a lot of time to fix these problems and could send stable devkits hardware way earlier to devs then XBox.