How does the raindrop attain constant velocity after some time?

The stoke's law states that  the viscous force acting on the body is directly proportional to its velocity. The velocity of the falling raindrop increases due to gravity acting on it. So, the viscous force also increases.
After a certain amount of velocity is attained, the weight W becomes equal to the upward forces. That is, the viscous force and upward thrust of the raindrop acquires a constant velocity.