Higher compression = higher temps = easier detonation =bye bye motor.
Shape of the pistons, how much is sticks out of, or under the cylinder walls, all determine the CR. ITRs have "more room" inside the cylinder during compression, which makes the ratio lower, and there for makes it a more suitable piston for everyday driving. Very high compressions require good fuel tuning to keep them in check, lower the ratio the less accurate it has to be.