That's a great question. I also have wondered from time to time why the non-adjustable stoptail was even used, and why not all stoptails be the adjustable type. Many customers I've spoke with have wondered the same.
I can simply give my 0.02$, because I don't really know why. My first guess is cost. To purchase the adjustable on its own is not cheap, and would leave me to believe they're expensive to make. Another guess is simply that Paul feels the standard stop sounds or operates better. He's always known to use the non-adjustable style. He also isn't known for down-tuning much, if at all.
Here's another thought, and again, a very subjective one on my part. Intonation adjustments seem to be an attempt to "fix" or "correct" inefficiencies in design or construction with regards to bridge, nut, and fret placement. To "compensate" for a lack of solid construction and reliability to keep the guitar from shifting parts around just slightly.If PRS can construct a guitar with such a low level of these inefficiencies, and be so consistent and reliable from guitar to guitar, then compensation no longer becomes a necessity!
When I started with my first CST22 stoptail, I spent the money and replaced the bridge with an adjustable version. I do different tunings and gauges from time to time, but mostly live in standard. I also replaced it because many guitarists I look up to (Tim Mahoney, Mike Einziger, Pete Loeffler) had their custom PRS's with adjustable bridges. Over a period of about 2 years, from being constantly used and sweated on, I completey disintegrated the bridge from dirt build up in between the saddles. They wouldn't even budge if I wanted to move them. So I went back to the regular bridge. I would say I actually perfer the regular one now. It doesn't build up dirt at all, it's super easy to work with, and it sounds and works great.
BTW, I happened to notice recently Tim Mahoney replaced his adjustable on "Ol' Blue" back to a regular stoptail. Why? Who the hell knows. He probably encountered the same problem I had from playing a billion shows a year.