A couple of reasons why.
In order to ensure maximum security your password needs to be strong enough, with at least 8 characters. Then, we make sure your password is not part of a list of common passwords that are easily hacked. We have an official list of 1,000,000 of these passwords that can be found worldwide. Your password should not be part of this list.
We recommend you use the password generator on the page to get a random secure password.