ما الحكمة من لبس القصير؟

The wisdom behind shortening the garment to mid-calf or above the ankles is to avoid pride (khuyala') or its causes, because extending it below the ankles falls into that category. Shortening the garment is purer for it and more reverent to Allah. Not all rulings of the Sharia may have an apparent reason, but the general wisdom is that they are a decree from the Sharia, as 'Aisha (may Allah be pleased with her) explained to Mu'adha regarding a menstruating woman making up for fasting but not for prayer.