Why? Because the size of the pins on pin base replacement bulbs depends on the wattage of the lamp. Therefore you have the same wattage bulb in order to replace it or it won’t fit. My friend recently purchased a 38W 2D pin lamp to replace the bulb that had burned out in her fixture. The pins didn’t fit. On closer inspection, she noticed that the old bulb said 55W. She went out and bought the 55W bulb and yes the pins fit in correctly! You don’t have this problem with screw in bulbs – they are flexible and you can select from a variety of wattages and shapes to go into the same socket.
So if your pin base bulbs burns out and you have to replace the pin base circular lamp say, make sure you’re replacing it with the same wattage. This is one of the disadvantages of pin based lamps.
There are other CFL bulbs in the market that don’t have this problem. GU24 bulbs offer more flexiblity in that you can replace the bulbs with different watt bulbs. Moreover, they are compact.