Because they don't want to, yet? Maybe because changing from dx9 to dx11, and everything that comes with it, is a much more difficult task than making a 3d cockpit*? Most of the engines that change APIs don't take to the transition too well, especially with smaller companies (Arma 3, almost non existent dx11 benefits, and Il-2 CloD, dx10 implementation was really buggy and almost no one uses it, come to mind in the sim world), the exact opposite of what happened with DCS.

* Not trying to say that BMS hasn't done some amazing work, they have.