Because if anything happens to your trucrypt password, the bitcoins are lost forever.  If you didn't write down the password, and you forget it you're hosed.  If you die, your family doesn't inherit your bitcoins.

So then you figure ok I'll write down the trucrypt password.  Now you're right back where you started with the wallet.

One solution to this that I'm using is Shamir's Secret Sharing algorithm.  It gives you redundancy (losing one of the printed keys is ok) but without the increased risk (an attacker finding one of your copies doesn't help him steal your coins).