The structure "It's very hot today, isn't it?" is a common way in English to make a statement about the weather or a situation and then seek agreement or confirmation from the listener. This form is called a "tag question."

In this case, "It's very hot today" is the statement or the main clause, and "isn't it" is the tag question. The tag question "isn't it" is added to the end of the statement to inquire whether the listener agrees with the statement or to seek confirmation.

So, the reason for choosing "Isn't it" to complete the sentence is that it forms a grammatically correct and natural-sounding tag question to seek agreement or confirmation about the statement that it's very hot today.