I've wondered this for a while, and there doesn't seem to be any real consensus anywhere that I can find. Some blame the lack of political willingness to kill and oppress as many people as needed to keep order. Some blame superpower rivalries - any territory you want to control, there's a rival superpower prepared to supply essentially unlimited numbers of weapons to any resistance force that cares to fight, and attacking the rival superpower directly is too dangerous to contemplate. It could also be that weapons technology - ready availability of bombs and firearms - makes it much easier for small groups with minimal training to inflict casualties upon a superior force. Maybe it's all three of them, and more reasons besides. I'd be interested if there's any way to conclusively prove or disprove any of them.