They probably also noticed that anyone interested in applying and has a realistic chance of admission (based on other factors) has an 800 on that subject test anyway, so it is not a useful differentiator in their applicant pools.
Given the preponderance of 800 scores, it is likely that many takers of this subject test did no additional preparation beyond completing the precalculus course at their high schools. Obviously, this does depend on the quality of the precalculus (and lower math courses) at the high school, but it is not like the content of this subject test is so far beyond high school precalculus that students need to assume that they need additional preparation beyond completing the course.