If you weren't hit front on, and you weren't moving, or not moving fast, it's most likely there was not enough G-force generated to fire the airbags. If the car hits a narrow object, like a pole, directly front on, between where the sensors are located, and isn't going fast enough, its possible for the airbags to not go off as well.

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F.B. Racing
01 WRX (STI badged)
2010 XR5 Focus
06 Impreza Hatch 2.0i