Explanation

You cannot go out because it’s raining very hard.

آپ باہر نہیں جا سکتے کیونکہ بارش بہت سخت ہو رہی ہے۔

The sentence explains why you cannot go out right now, so we need a verb that shows a present continuous action.

“It’s raining” = It is raining → Present continuous tense

It matches the context of an action happening right now.