I don't know. I have many friends that regularly do halves. I know fewer that do a marathon for than once. A half marathon is not an easy thing to do. It requir

I don't know. I have many friends that regularly do halves. I know fewer that do a marathon for than once. A half marathon is not an easy thing to do. It requires preparation and training -- obviously not as much as a full marathon, but still quite a bit, especially if you're not someone who runs regularly. There's always something harder or more prestigious to do -- for many people a half marathon is enough of an accomplishment that they don't feel the need to progress, in the same way that many feel that many think a marathon is enough and they don't need to run an ultra.