Apparently Hasan switched sides. One Obvious possibility is that some lasting fear about the inclusion of Muslims in this country and especially the Military, could come from it. Along with this comes the fear of unwarranted discrimination and other reprisals Muslims must have in the U.S., including in the military . It has probably brought on more fear of reprisals than anything else.
It is possible that those who want an immediate and high profile public conclusion that Hassan was a terrorist may not get satisfaction based on sensitivities to these points. My feeling is that the restraint is good.