Because nearly everyone involved in Sandy Hook thought they were playing set roles in a drill. Sirens and emergency speeding are inherently dangerous and for that reason are not used in drills - just like live ammunition. Those running a drill - DHS, FEMA, State Police would be liable for any injuries, death, and destruction.

I think Sandy Hook was a long-term, well-planned drill that threw the leading agencies into utter panic when at some point their drill "went live". Unlike fatalities caused by a speeding emergency vehicle the liability to the state of gunshot victims was unfathomable and unacceptable. I think this is the reason no EMTs were let into the building and no victims were brought out in daylight.