Williams killed the line around 95, at least in print, as part of her purge of Gary Loyalists, and Mystara and Greyhawk were part of the purge. But Mystara still kept going because Capcom got the license to make Shadows Over Mystara, which turned out to be one of the best beat 'em up's ever made. The fact they repackage the game every so many years for the next generation of consoles keeps it pretty much in constant use. Mystara is still well known because of those games, which actually meshed well with 5e's superheroic style of play.