I think an electrical engineer might better answer that one but from what I understand it's because the grid is interconnected. Power grids all over the world are interconnected. It evens out the load distribution. The bigger the interconnected area the more efficient the power production.

It's not easy to sabotage just one country in complete isolation of ones next to it. You would have to involve more and more people to do it who would have to prepare in advance for the sudden power drop.