Because that was part of the DNC script. This thing was supposed to go off, with this guy in the car. The plan was to blame the "right-wing" tea party people, in order to sway public opinion. The poorly-made bomb failed, the suspect was spotted fleeing, and the plan was aborted. But the state-run news media didn't get their new marching orders in time, so they continued to blather on about those "tea party people".
/tin foil hat...