Question

why melting of ice causes of cooling

Answer 1

For ice to be used as a coolant, the ice - even if initially at 0°C - has to first melt, and that by itself absorbs a large amount of thermal energy because of its large latent heat of fusion. And once melted, it is still at 0°C, just  ...

Answer 2

As the ice is melting, the temp. of surrounding would surely be more.
And hence greater than 0 Celsius. So, to melt, it absorbs heat and cools the surrounding.
It's that simple.