Mostly because it's one of those rural roads that actually leads somewhere of note. And my view is in that, in the most rural areas, most roads that are drivable by any vehicle and actually lead to some significant points of interest (e.g. a popular trailhead or reservoir access) should often be tertiary. That's in contrast to roads that need higher clearance or roads that just lead to a farm or a few houses or the like, or roads that are not drivable in a low-clearance 2wd sedan, which I would definitely leave as unclassified or residential depending on use. To me this makes sense to indicate which roads people are actually slightly important to the general public rather than just roads that nobody would have a reason to take.

But I agree with you that this one is borderline, since it's certain rural but not quite the middle of nowhere like many other similar roads in Utah. If someone were to change it to unclassified then I wouldn't complain.