In the United States, the most famous Spanish dramatist of the 20th century is Federico García Lorca, so why doesn't he have a major memorial in Madrid?
Well, there is the matter of Lorca being murdered by his own government, but the larger issue is that those writers who ultimately gain the most respect internationally are not necessarily the ones who are honored in their home countries.
That's not to say that Benavente wasn't recognized outside of Spain. In fact, he won the Nobel Prize for Literature in 1922. But Bjørnstjerne Bjørnson also won that prize, while his fellow Norwegian dramatist, Henrik Ibsen, did not. Momentary recognition does not always translate into lasting fame.
So who was this Benavente guy, anyway? A native of Madrid (unlike Lorca, who was born outside of Granada), Benavente was known for writing in a Realist style. His 1913 play The Unloved Woman has been repeatedly adapted to film. An earlier play, The Bonds of Interest, is frequently cited as his masterpiece, and is his most frequently staged work.
But is it staged as frequently as Lorca's Blood Wedding or The House of Bernarda Alba? Probably not. Sometimes, life just isn't fair.