Why should gases be insulators at ordinary pressures and start conducting at very low pressures?

At low pressures, uner the effect of high voltage ions acquire sufficient energy; have a chance to reach their respective electrodes and constitute a current.

At ordinary pressures, positive ions and electrons produced by ionisation of gas molecules do not reach the electrodes even at higher potential. this is because, of the frequent collisions of molecules and their recombination. Hence, at ordinary pressure gases act as insulators.