Why would Mendix prevent startup of a service if somebody in the DB with admin roles have set their password to 1, but 2 is okay? The problem with the service not starting up, is if you are in production you first need to gain access to the SQL server and then manually set the password. So you can't use Mendix's Users menu to set the password. Perhaps validating the password is a more graceful approach?
To answer your question, because 1 is a simple commonly used testing password and 2 isn't. It isn't necessarily meant as a security precaution, but just to make users aware of the fact that they're running in production with an insecure password.
But you can set your password before starting up both in *nix and Windows environments. Refer to the deployment documentation.