Much ink has been spilled over the difference between sentences like “It is I” versus “It’s me.” What this matter comes down to, of course, is the coherence between stylistics and grammar, in case there is a choice. That is why in contemporary English, whenever a speaker chooses to utter the answer to the question, “Who is it?,” will use the objective form of the pronoun with a contracted copula (i. e., “It’s me”) and the subjective (nominative) form of the pronoun with the full (uncontracted) form of the copula (i. e., “It is I”, as stilted as this may sound in contemporary speech).
The underlying reason for this particular outcome has to do with the stylistic value of contraction. As between contracted and uncontracted forms, contraction always involves the colloquial (informal) stratum of the linguistic means at one’s disposal, while the uncontracted form is coherent with the formal stratum. Hence the variation of the form of the copula in the construction at issue.