Glad to hear you're thoroughly enjoying the 3-20 Nate!
OP, the general consensus on the Ultra Shorts is that they are truly amazing scopes for what they are - very short for their mag range and objective size. It is harder, from an engineering and manufacturing standpoint, to make ultra short scopes and even harder to make them "light" - with light being an objective term: If you compare the 3-20 Ultra Short to Vortex's Gen II 3-18 you'll see that the Schmidt is much lighter, compare it to Bushnell's 3.5-21x50 DMRII and it's still lighter, compared to ZCO's 4-20 and it's lighter, compared to the Kahles K318i and it's lighter but compared to the Nightforce ATACR F1 4-16x42 it is a fraction heavier, compare to the Leupold Mark 5 3.6-18x44 and it's heavier and compared to the Leupold Mark 6 3-18x44 it's heavier or the March 3-24x52 and it's heavier. With exception of the March all the scopes that are lighter than the Ultra Short 3-20 have smaller objectives and do not have wide a range of magnification and are not on par optically, the March may be the closest but struggles with a finicky eyebox and parallax as well as a lack of depth. The only scope I've found to be on par (and slightly exceeding) the Schmidt optically is the Kahles K318i, but this scope has less FOV, less magnification and is heavier but it is much shorter - to its benefit. The Premier LT or TT315M may just barely qualify as an Ultra Short and the Hensoldt 4-16x56 may as well, both these scopes may perform better optically vs. the Schmidt but have their own drawbacks (Sorry if I left a scope out, the good news is there are quite a few options available now in this category).
I sold my Schmidt US 3-20 last year mostly because I preferred the SKMR style .2 mil reticle over the MSR (reticles are personal preference); however, I have a Schmidt Ultra Short 3-20 with MSR2 reticle due from UPS tomorrow and am hoping it will impress me - I've had numerous discussions with FinnAccuracy and think I will really like all the enhanced features over the MSR. I'll be doing a quick analysis of the Kahles vs. the Schmidt so stay tuned.
I believe the reason we don't see more 3-20's for PRS/NRL is because optics tend to struggle at their extreme ranges and if you're in competition you are likely to want the best optical quality for the sweet spot. A few months ago there was a discussion similar to this where there was a debate on what is the most common top magnification used in competition, by a fair margin the consensus was that 15-20x seemed to be the max for most competitors even at long range. Frank had joined the discussion and mentioned he uses Schmidt's 3-27x56 scope and I asked if he never used it above 20x for competition then why not use the 3-20 Ultra Short and he said it was because the 3-27 performs better optically in that 15-20 range than does the 3-20. Now that's not to say the 3-20 performs poorly, but there are optics that perform better and competitors tend to seek out that "edge" that can help them win. Another reason may be the high cost of Ultra Short optics, these alpha class scopes tend to be more expensive than their 5-25 counterparts with the Schmidt, Kahles and ZCO all going for north of $3k. Price, quality and durability play a big factor in the choices made for competitive shooters.
