The subject of this sentence is 'It'. The verb is 'is'. The predicate to the verb 'to be' should be in the objective case to show that it is not the subject of the verb. The objective case of 'I' is 'me'.
Consider it this way round:
You and I are it.
In this example, the 'I' is part of the subject. Therefore, it is written in the subjective case (I).
Many will say 'It's you and I', thinking they are correct, but I'm afraid, they're not.