Why the electrical discharge through the gases could be observed only at very low pressure and very high voltage

Due to the electric field, the gas gets ionised, releasing electrons which move towards the anode, thus constituting the electric current. A low pressure is required as at high pressure the movement of the electrons will be retarded due to collisions with the molecules/atoms of the gas

the passage of electric current through a gaseous medium under the action of an electric field, accompanied by changes in the state of the gas.

Electric discharge in gases occurs when electric current flows through a gaseous medium due to ionization of the gas

As the applied voltage is increased, the free electrons carrying the current gain enough energy to cause further ionization