We have to study Aristophanes' Lysistrata for Drama, and I can honestly see no reason why anyone would bother staging it. Whilst Greek tragedy has been terribly influential on drama and prompted some great plays, Greek comedy (there is only one playwright's works surviving but still) just isn't funny. The translation is terrible- it reads like a bad Carry On film- and aside from that, it's boring. There isn't a plot and there's too much formal verse and convention for anyone who isn't a historian or theatre buff to find funny. Theatre's moved on, people have more sophisticated tastes, comedy's moved on.
The only thing it has to recommend it is an anti-war message but there are far better plays with that.
Can you think of any literature that now seems irrelevant?