Question

Why electric discharge is favoured at low pressure

Answer 1

It doesn’t. A sufficiently high voltage will break down any gas at any pressure, or for that matter any liquid or solid. It’s just easier at low pressure, because the electrons torn loose from one atom or molecule get to accelerate over a longer distance (on average) before they hit the next one, and if they hit with enough energy they will knock loose additional electrons and form a cascade discharge.

At sufficiently low pressure (ultra high vacuum) it gets harder again, because the liberated electrons don’t hit anything before they get to the other electrode — or they have so few collisions that breakdown isn’t completed.