Because during lightning an electric current passes through air. poor conductors also allow electricity to pass under certain conditions
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YES, but what u mean by UNDER CERTAIN CONDITIONS????
i some conditions 
air too conduct electricity as pure air is the poor conductor but in lightning it contain a certain amount of dust and other particles that's why it conduct electricity
Air particles also contains electrons but in less amount therefore it act as insulator. When thunder lighting pass through air most of it's energy is wasted due to insulation. The part of light we see is the conducted part of electricity which has less energy than the energy produced on it's occurrence.
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