Asymmetric cryptography is harder to use. Each person who wants to use asymmetric cryptography uses a secret number (a "private key") that is not shared, and a different number (a "public key") that they can tell everyone. If someone else wants to send this person a message, they'll use the number they've been told to hide the message. Now the message cannot be revealed, even by the sender, but the receiver can easily reveal the message with his secret or "private key". This way, nobody else needs to know the secret key.