Here's why:
* Declarative sentences state a fact or give a command. Orders are a type of command.
* Periods signal the end of a sentence, and therefore, the completion of a thought or directive.
Examples:
* "Please close the door." (This is a polite order.)
* "Go to your room." (This is a direct order.)
Other punctuation marks can also be used in conjunction with a declarative sentence to emphasize an order:
* Exclamation point (!) can add urgency or intensity to an order. For example, "Stop! You're in danger!"
* Question mark (?) can be used in a rhetorical question that implies an order. For example, "Don't you want to help me?"
However, the period is the most straightforward and unambiguous way to punctuate a sentence that gives an order.