From what I understand it’s a question of getting the atoms closer to each other so electrons have less gap to jump. Cooling down and high pressure both contract materials so that’s why they are used to create a super conducted state. Apparently the all-99 has atoms structured in a way that they’re really close to each other in ambient state.  This was not in the video, I read it elsewhere.
Edit: it’s in the Wikipedia article. https://en.wikipedia.org/wiki/LK-99#Proposed_mechanism_for_superconductivity
Copper ions are smaller than lead ions and take less room in creating almost 50% reduction in volume.