I've never understood why this is. For about 50 years- from the latter part of the Elizabethan age to the Civil War- there was a great outpouring of excellent plays in verse- not only by Shakespeare, but by Marlowe, Jonson, Middleton et al. There are so many good plays from this period that most of them are sadly neglected. Revive a near-forgotten piece by Dekker or Fletcher or one of those other nearly-nearly men and the likelihood is it'll go like a bomb- and you'll have a huge success on your hands. For that brief period the writing of good plays in verse was easy. Anyone could do it. A brothel keeper called Wilkins thought he'd try his hand, got a little help from the resident playwright at the Globe- and the result was Pericles. Then the Republic shut the theatres and no plays of any kind were performed for 15 years. When Charles II returned in 1660 the flow of verse plays started up again- and none of them was any good.
Elsewhere this wasn't the case. In France the later 17th century is the age of Moliere, Corneille, Racine. The German were writing great verse plays in the early 1800s.
And it's not as if the theatre went into decline. The Restoration theatre was lively, the 18th century was lively, the 19th century theatre was lively. David Garrick- arguably the greatest actor of all time- knew all the poets of his age and they all wrote plays for him and none of those hundreds of plays is still performed. You'd think the culture of the coffee house- Garrick sitting down to trade wit with Johnson and Goldsmith and all that gang- would have resulted in something lively and living- but it didn't. Well, a few prose comedies, but not a single play in verse that has remained in the repertoire.
Maybe the shadow of Shakespeare killed off the fresh green shoots. Coleridge said he tried to write Shakespeare and it came out Massinger.
Coming into the modern era there's the odd glimmer. Murder in the Cathedral is pretty decent and I've always thought Eliot might have got somewhere if he'd continued in the vein of Sweeney Agonistes instead of trying to merge Aeschylus with drawing-room drama. Under Milk Wood is still performed, though it's a radio play not a theatre piece. The Lady's Not For Burning and other plays by Fry get revived occasionally. But would you say that any of these poets was a great dramatist? I don't think so.
The last 50-60 years have been a theatrical golden age- but has anyone written a verse play that will last? There have been contenders. Tony Harrison for example. But I don't think any of them have pulled it off.
And then there's Yeats- who started me off down this track in the first place. He, like Shakespeare, was totally a man of the theatre. He ran the Abbey, he hung out with actors, he had colleagues who were writing plays- but not verse plays- that have lasted. He was a great poet, he had the ambition, he was in the right place at the right time- and he failed.
No, I don't get it.