Why do I get weaker when I stop taking creatine?

Creatine increases your muscle's capacity to perform work. So, when you go off creatine, you have less capacity. That simple!

Think of it as a tank of gas in your car. With a full tank, you can drive farther than with a partially full tank before you need to gas up again. Creatine keeps the tank more full, so you get more mileage before gassing up (i.e. ending the workout and recovering.)