When electrons move through a gas, they collide with the gaseous particles and lose energy. This increases the resistance and, hence, reduces the current. But at low pressure, as the gas particle are widely spread, there are fewer collisions. So, the electrons can pass easily and with less collisions.If the pressure is reduced to nearly zero, the current through the gas will decrease. This is because the mean free-path of the electrons (distance that the electrons travel between collisions) is longer and they can, therefore, be accelerated to higher speeds before collision with an atom and they have more chance of causing ionisation.