Here's why:
* "It's" is a contraction of "it is" or "it has". Contractions are shortened forms of words and phrases.
* Possessive pronouns are used to show ownership (e.g., mine, yours, his, hers, its, ours, theirs).
The possessive pronoun for "it" is "its." For example:
* The dog wagged its tail.
* The tree lost its leaves.
In short, you would never use "it's" as a possessive pronoun.