Perhaps because they didn't get that specific part of the genetic code. Heck, if anything, regeneration strikes me as more of an epigenetic process, a mechanism that repairs the genes and modifies their expression. The DNA without the epigenetic component wouldn't be able to regenerate any more than a car could repair itself without a mechanic. Or maybe regeneration isn't a purely internal process. In "The Power of the Daleks," the newborn Second Doctor explained the process of "renewal" as "part of the TARDIS." And all of the Doctor's regenerations that we've seen have happened either within the TARDIS (the first, fifth, sixth, ninth, and tenth regenerations), immediately outside it (the third), or at least within the same general area (the second, fourth, and seventh). It seems likely that the eighth regeneration happened in the TARDIS as well, since at the climax of the Time War with Gallifrey destroyed, where else could the Doctor have been and survived?