It is true that most of Handke's plays were written decades ago, but it was his plays that made his name, and they continue to be performed. (I recently saw a video of a production of Offending the Audience performed by children!) And in Europe Jellinek is mostly known for her plays.
Stoppard is a terrific commercial playwright, but the Nobel tends to honor writers who push the boundaries. It went to O'Neill not Williams. t went to Beckett not Miller.
As one friend said, even if the award is international, you will not ever see someone win whose work is not available in any Swedish bookstore. And given O'Neill's popularity in Sweden maybe its not a coincidence that he is the only American playwright to be honored.
Is Stoppard much performed outside the English speaking world? (That is a genuine question--I do not know.)