I am fairly sure this is because, in many abugida scripts, the dominant writing material was palm leaf and the tool was a sharpened point or curved scalpel-like knife rather than a pen. If you curve inward from both sides, you don't have to contort your hand in a 180 degree pattern, which runs the risk of tearing the palm leaf. This is also why the shapes are all curves instead of hard 90 degree angles.