Because when a Diners Club (or other partner) member turns in Diners Club points for DL SkyMiles, Diners has to purchase those DL miles from DL. The cash goes into DL's pockets. Meanwhile DL might not have to pay off on those miles with an award for many years. And even when it does, it's likely to be with a seat on a plane, not with cash out of its pocket.

Cash is King!