The answer is sweptback wings are optimised for high speed, high altitude cruise flight. At slow speed, they need extensive lift devices to produce more lift. This means leading edge devices and trailing edge flaps to reduce what would otherwise be a high stalling speed and create more low speed lift. The result is a higher nose attitude at the slow approach speeds. The effect of having no extra lift devices operating can mean approach speeds increased by 50 knots for a B737, and danger of tailscrape because of attitude.