Why didn't these scientists restore electrical activity in this pig's brain?
Restarting electrical activity would mean that at least parts of the brain become active again and possibly restoration of almost all cognitive functions. While this would be a tremendous scientific and medical break-through, it would also mean that effectively locked-in syndrome would have been induced for these pig brains. Therefore just attempting to restore electric activity in isolated brains has huge ethical problems and was for this reason probably not allowed (or ever attempted).

Whether or not it should be allowed to test this in pig brains (in order to learn how to adapt the procedure to humans) is a different question (and not one for this SE) - the best way to go about would probably be to restore electric activity not in isolated brains, but with the brains still in animals.