Why we drive cars slowly in rainy season?

For lots of reasons. Wet roads are harder to grip on and easier to skid on, so you need more space between cars and travelling fast will increase the distance you skid. You could go off the road easier in wet weather. Visibility isn't as good in wet weather, so it is better to drive slowly. Water can gather in anything from puddles to large floods, and these could affect the car. It is harder to know what the surface of the road is like, so you need to drive carefully. For these and other reasons, you need to slow down in wet weather.