2 big reasons for that:

1. Fits better between studs as the other commenter stated
2. Easier and cleaner to route the 2 power phases. US plugs are famously ~120V but what many don't realize is that's a single phase of 120V, and there's two phases that go into the breaker box. By combining the +120VAC and -120VAC phases you get a full 240VAC for higher power appliances like stoves, dryers, heat pumps and electric vehicles.