I think it's because the Cadmium-Sulfide (CdS) photocells in light-sensitive switches fluctuate in value over time and can go bad. The resistance of the cell goes up when the ambient light goes down and if the photocell is broken, it becomes a very high value resistor which basically looks to the control circuit in your photosensitive switch like it's always dark. The photocell itself is a $0.50 item but it's usually an integral part of the control switch. You may need to replace the switch unless you're familiar with soldering and electronics. The switch would run around $10 or less, not a terribly expensive item either.
It usually looks like a small box with a round window or a small cylinder connected to the light fixture with three wires. They also sell switches that screw into a normal light bulb socket, and then you screw the bulb into the socket on the switch itself - it may be an even easier repair if space in your light fixture allows it.
Every hardware store, most certainly big box stores sell all types of photosensitive switches.Â