Question

why do the coastal areas of India experience less contrast in temperature

Answer 1

The simplest answer is that" due to the moderating effect of sea".
we know that coastal areas means having a sea . We also know that the water specific heat capacity is much higher than the soil as well as the rocks.
So it is difficult to change the temperature of ocean rather to increase or decrease it as compared to land. This is why we see little variation of temperature in the coastal areas.

In order to understand  more simply keep in mind that, water is difficult to warm or cool than the soil or rock . Which is why temperature variation is not too much on coastal areas.

Answer 2

The coastal areas experience less variation in temperature as compared to land. The reason is that the heating or cooling capacity of water is much less as compared to land. It means that water takes more time to get hot or cold then  land. Therefore areas near sea or ocean have more moderate temperature in the day time and less cooler at night. In the day time, as the water takes comparatively more time to get heated, so areas around water (coastal areas) remains moderate. Similarly, at night water takes more time to get cooled hence areas around ocean do not show much variation in temperature.