Compression is ultimately what makes power in an engine. The more you can compress that fuel/air mixture, the more powerful the ignition will be. The reason a boosted engine requires low-compression pistons is because the turbo (or supercharger) creates boost. The low compression ratio is needed because, if you had a high compression engine with a turbo, the compression ratio would go so high that it would destroy the engine in a very short time.
With a naturally aspirated engine, there's no boost except for what the pistons generate in the compression ratio. So, the idea for making all-motor power is to get the compression ratio as high as possible without causing pre-detonation (spark knock). Think of it this way...in a turbocharged engine, compression ratio changes as the turbo forces more and more compression into the cylinders. The harder the engine pushes, the more boost is created, and the more power is made (with proper tuning, obviously). In an all-motor engine, the compression ratio is exactly the same at idle as it is at 8,000 RPM, give or take a few PSI. Make sense?