Maple syrup was originally a locally grown (American) sugar substitute. As such, what was valued was a neutral flavor, i.e., something that tasted as little like maple as possible, and as much like sweet liquid. Thus, Grade A (and Grade A Fancy, etc.) went to the lightest, mildest syrup, and Grade B (and Grade C, which is for industrial use) went to the darker syrup with a stronger maple flavor.
(As an aside: the grading is done by color. There is a small kit producers buy which has small samples of colored liquid for each grade, and the grade is assigned based on which liquid your syrup's color matches most closely.)
The change in preferences came when cane sugar became a cheaper commodity; there was no longer any reason to use maple syrup (or maple sugar) if you weren't looking for the maple flavor.
I used to make my own maple syrup, back when I lived in the US-- it was a lot of work, but a lot of fun.