It's a packaging issue. Boxers are excellent for weight distribution, but their width would restrict airflow to the rear diffuser. V engines fit neatly behind the cockpit, with plenty of space for ancillaries. Rear aerodynamics are a key aspect of Nissan's decision to go front-engined; putting the engine up front allows for an absolutely huge diffuser tunnel.

https://s-media-cache-ak0.pinimg.com/736x/1a/4a/73/1a4a73715...

Also because it's a stressed member, which is significantly easier to build in a V layout than a flat layout.

The lower CoG thing with flat engines is kind of a myth. You end up with the exhaust headers below the engine, so the engine sits higher than an equivalent V layout engine would.