Why shouldn't you use an apostrophe for the pronoun its?

None of the possessive pronoun forms use an apostrophe:

• the possessive pronouns are: mine, yours, hers, his, its, ours, theirs
• the possessive adjectives are: my, your, his, hers, its, our, their

The Word its with an apostrophe is the contraction it's, a shortened form for it is. The Word its and the Word it's have two different meanings, two different functions:

• The puppy got its paws muddy.
• It's time for Monday night football!