All that granted, and I'm aware of it. Nonetheless, it is in the context of the season it was in that is my point. Despite the awards it won that year, or the filming of it, or its run, it is not, at least in what I have heard and read, an especially respected or beloved musical. And the feeling I remember at the time was that it was just good enough to win that year, in part because it was original, was doing well and was at least decent, but the aura of greatness does not and never did seem to emanate from this show. (And it is not especially celebrated or obsessed over by musical theater fans, from what I can tell.) That's what makes me say it one of the weaker Best Musical winners. It joins (original) winners like Gentleman's Guide, Will Rogers Follies, Raisin, and Two Gentleman of Verona in this category.
I guess with Memphis, time will tell moreso (though it has been almost a decade).