The reason the verb "to be" is an exception is that its meaning makes it equivalent to an equal sign. "It is I." means: It = I.

Both "It" and "I" are co-equal subjects of the sentence. There is no object. The subject of a sentence, in this case both subjects, require the nominative case.

Contrast this with the sentence : "It hit me." The subject "it" acts upon the object "me," so the objective case is required.

Another example of the exception with the verb "to be", which may be surprising, is: "It was we." This is the correct usage for the same reason, however in common usage, most people say, "It was us," which is technically incorrect.