Think of it in terms of an energy barrier, like someone cycling over Everest. It might be theoretically possible that there's a nice happy superconducting state

Think of it in terms of an energy barrier, like someone cycling over Everest. It might be theoretically possible that there's a nice happy superconducting state if you can get things in the right place, but if you've got to cycle over the top of Everest, it might be pretty much impossible to get there.