Sometimes less is more. Not everyone wants to run the highest magnification possible, even when it more magnification may be available on their scope. As mentioned by Kraken, the field of view becomes narrower. This can sometimes be a problem in competition depending on the size and placement of the target number boards. A crossfire is far worse than possibly dropping a point or two because you couldn't hold as tight as you wanted due to the lower mag setting. The effect of mirage can also be "amplified" by increasing magnification, and turning the magnification down will sometimes allow a clearer sight picture. With a variable magnification scope, these issues are usually not such a problem; you can adjust the magnification as desired. With a fixed power scope, there is no choice, whatever the magnification the scope has is what you have to use. Apertures that reduce the objective lens opening diameter will sometimes help with mirage, but they can also noticeably reduce the amount of transmitted light.
A couple years ago, I purchased an IOR-Valdada fixed 36X BR scope for an F-TR rifle. I usually run a Nightforce NXS 12-42x56 or Competition 15-55x52 scope at around 32X in matches. If the mag on the IOR-Valdada were any higher, I would not be able to see much of the target number boards at my home range and crossfires would definitely be the result. The 36X magnification is sufficient that I can easily hold a little better than halfway between scoring rings, or hold on one side or the other of the X-ring (i.e. under 0.25 MOA). More magnification wouldn't really be an improvement for my eyes because I don't believe that I, or my rifles, are capable of better accuracy/precision than that, even if I could hold tighter. Much of that would depend on the particular discipline or the intended use. I can imagine that higher mag would be useful in some disciplines, but perhaps Sightron has chosen to reduce the magnification as a business decision based on customer feedback. If so, you might imagine the majority of shooters using their fixed power scopes suggested a reduction in magnification might be of benefit for their particular discipline or intended use.