Please discuss here all your remarks and pose your questions about all racing series, except Formula One. Both technical and other questions about GP2, Touring cars, IRL, LMS, ...
Technically, very easy. But there are two major handicaps in this possibility happening. First, there's the economic and poltical aspect of the engine supplier. All cars are spec, with just one chassis and engine type. The system is working very well right now, why screw it up?
Secondly, these cars are designed to mimic Formula One cars as much as possible. The external dimensions are the same, and the aero is quite similar. Wings front and rear, a bit of underbody generating downforce. And one thing a chassis likes is a narrow engine configuration, to allow rear diffusers. If you build in a flat engine, that would have a huge negative impact on the chassis being able to generate downforce, which would move these cars further away from F1 cars, which is in opposition to one of their agendas.
Additionally, a flat engine is a nightmare in racing applications. In the interests of lower center of gravity, it's desirable to have the crankshaft, thus the engine block as low as possible. But with a flat engine, you have to give room for the exhaust, or intake, whatever you choose to build on the bottom of the engine. That gives you an engine that resides much higher than desired.
