Without having any direct contacts on this, my guess would be that it became too expensive because the audience it was seeking was too small or just not there. I, too, have every edition and continue to revel in them frequently. Pray that the same fate does not befall Theatre World, even though the editions come out way too far after any given season. 70 years ain't bad, though, and I have every edition since Volume One. Broadway (and Off) has a limited audience appeal, with relatively few shows making that a great impression. Tourist trade has keep the Wickeds, the Lion Kings, and the other mega-hits alive and flourishing, but only the tri-state area audiences keep the majority of other shows going for a while; when that audience dries up, the show closes. Wish it were otherwise.
BroadwayEd