Every few kilometers, one would need liquid helium pumping and chilling plants.
Total helium losses to leakage wouldn't be too big. Electricity transmission efficiency would be reduced by all the chilling gear running, but still better than regular conductors.
The only real barrier is cost. Not even the cost of the superconducting material. You can't hang a 6 feet diameter pipe on pylons across the nation - you're going to have to bury it, and that's going to get exxxxxpensive fast!
I feel like this could almost be a slogan for the human race at this point. Our grasp of science and engineering has reached a level where's there's very few things we might want to build where we couldn't conceivably do it. We could build the most outlandish megastructures, infrastructure and space bases if we really had to, but it all just costs too much.
We might be able to anything, but the capitalist system stops us doing things that don't look profitable.
Where would we get enough helium to cover more than a small city's worth of power transmission lines?