Mainly because it is impossible for anyone to take your bitcoins without your consent (if you know what you're doing, of course).

It's not like you can place a password on your gold to prevent authorities from seizing it as they have done in the past.

I'm not sure it's any more resistant to rubber-hose attacks ("What's your private key/passphrase?" vs "Where did you bury your gold stash?"). Perhaps a little more convenient, though.