I've heard for years that this Greatest Hits package was the best selling album of all time (and I hadn't heard that, at one point, it was passed by Thriller), and I've often wondered why this album had such selling power. Obviously, it had the advantage of having been released well before the CD era, but so was Thriller and lots of other albums that I think a lot of folks would have thought were more popular.
Like Stadler, I don't understand the distinction between "casual music fans" and "real music" fans (and the follow up explanation doesn't help, but I won't pile on). Also, having the best selling album does not make the Eagles "the most commercially successful American band of all time." Even if only album sales are considered (which would be dumb if considering "most commercially successful . . . of all time"), the Eagles come in around #10 on that list.