This compilation of Time Magazine covers about authors also includes playwrights, of which there are, perhaps predictably, even more scarce than the novelists. Only five--five!--made the cut: Eugene O'Neill, George Bernard Shaw, Noel Coward, Thornton Wilder, and Tennessee Williams, the last of which was in 1962. Somewhat freakishly, O'Neill made the cover a four times.
I'm sure some of the authors listed wrote a play or two, but as far as full-on dramatists go, it's a pretty dismal showing.
(h/t Sullivan)