I think I _can_answer this.
The reason the possessive of the pronoun "it" is spelled without the apostrophe is because
of its and the other, related, pronouns' history. When they were first introduced from Old English
they were all declined (in the various cases). Hence were formed the possessive pronouns:
his, hers, ours, theirs, etc. It's to maintain consistency with these that "its" is spelled
 without the apostrophe