That's because clouds can act like a blanket. The clouds can keep a good chunk of sunlight out during the day, keeping us cooler in the afternoons. But the clouds will also trap in whatever warmth we get during the day, keeping us warmer at night.
Thus, after a sunny and warm day where we might get to 55 during the day, a clear night will allow that heat to be radiated out back into space, dropping temperatures near freezing. While a cloudy and 45 degree day might only get down to the upper 30s at night.