Why do rain drops fall with small velocities eventhough they are falling from great heights?

The rain drops acquire a terminal velocity as they fall through air. This is  the weight of the drops is balanced against  the buoyant force and the viscous force that act in the upward direction. Once these forces are balanced, the net force on the drops is zero. So  their acceleration is zero and they fall with a steady velocity that is called the terminal velocity.

Normally when you consider a stone falling you neglect the viscous force on it due to air. You also neglect the buoyant force due to air. But in case of raindrops these forces cannot be neglected.