[Melkor89]

There are three reasons why Sauron may have made a ring of power. First, It had to be something. Why not a ring. Also, the elven smiths made their items of power rings. Since Sauron made his ring to rule the others and to bind their fate with it, perhaps the ring could only control another ring. Lastly, it is pore practical. A ring is small and can be easily carried anywhere. Also, in battle, something looped around a finger like a ring is more difficult to remove than say, a sword of power would be.