I've explained this at more length elsewhere in this thread, but the short version is that maple syrup (and maple sugar) were originally a substitute for sugar (which was more expensive, and could not be grown locally in North America), so the "better" grade was that which was most neutral, and had the least "maple" flavor. Once sugar became cheap enough, maple syrup began to be used only when one wanted the maple flavor, but by then the grading was well-established.