Complex sentence: The airlock was necessary because Soviet electronics used vacuum tubes and required air cooling, thus the capsule was depressurized for the EVA and it was impossible to re-pressurize the cabin again as there were insufficient supplies of oxygen and nitrogen on board.
Split into simpler sentences: The airlock was necessary because Soviet electronics used vacuum tubes and required air cooling. If the capsule was depressurized for the EVA, it would impossible to re-pressurize the cabin again as there were insufficient supplies of oxygen and nitrogen on board.