Why Not Twentieth Century Historicals?

Why are so few romance novels set in the twentieth century?If you go to a bookstore and look at the section labelled "Romance," certain things are there and certain things are not. With the exception of occasional outliers like Harlequin's line of 20th century "decade" books-- a line of categories that some bookstores didn't carry, and which was then closed down--historicals seem to include only