Why is this the case when the rules are so simple?
An apostrophe can show possession, eg: John's shoe; London's historical buildings; the peacock's glass eye.
If the word is plural, put the apostophe after the 's', eg: all the actors' voices could be heard; The Mothers' Union.
The exception occurs when the word is already a plural. Then you keep the apostrophe before the 's', eg: the children's coats; the men's room.
An apostrophe can also show omission, usually the omission of a letter. So, when 'it is' becomes it's, the apostrophe is simply showing where the omitted 'i' used to be. 'Hadn't' has lost an 'o' and so on.
The confusion seems to creep in with 'its'. This is a word in its (!) own right, without an apostrophe. It shows possession again, eg: I love this pink rose because of its beautiful scent.
If I had added an apostrophe, the sentence would have been nonsense, because it would have read: I love this pink rose because of it is beautiful scent.
Possession or omission. It's as simple as that. Isn't it?