Of course you are right that the huge increase in ticket prices (even adjusting for inflation) has a tremendous amount to do with it, but I think that's in combination with the fact that a much larger percentage of the Broadway audience now comes from outside the NYC metropolitan area than was the case back in the day. If people are only visiting the city for a very limited time, they have less chance to see multiple shows, so their first choice(s) will likely be the shows that are big events because they are spectacles or brands and/or have big stars in them.
I think it's this double whammy that has all but killed off light comedies without stars and other plays (and even some musicals) that are not viewed as "event" shows in one way or another.
