Why doesn't the 'su' command work in Ubuntu but does in other Linux distros?

Because in Ubuntu, by default, root has no password.

Without arguments, su means "switch user to root" and to do that you have to enter root's password, not your own. Unless you give root a password, it doesn't have one, so you can't literally log in as root.

Here's some background:

In the Unix world, there has always been a larger division between regular users and administrators, owing to the multiuser heritage of Unix. The approach taken in Unix is to grant superuser privileges only when needed. To do this, the su and sudo commands are commonly used.

Up until a few years ago, most Linux distributions relied on su for this purpose. su didn't require the configuration that sudo required, and having a root account is traditional in Unix. This introduced a problem. Users were tempted to operate as root unnecessarily. In fact, some users operated their systems as the root user exclusively, since it does away with all those annoying “permission denied” messages. This is how you reduce the security of a Linux system to that of a Windows system. Not a good idea.

When Ubuntu was introduced, its creators took a different tack. By default, Ubuntu disables logins to the root account (by failing to set a password for the account), and instead uses sudo to grant superuser privileges. The initial user account is granted full access to superuser privileges via sudo and may grant similar powers to subsequent user accounts.

~ The Linux Command Line

You can use su to switch user, if you know the password of the user to switch to, for example, I can su pixie to log in as my backup user.

Normally though, you should use sudo and your own password to authenticate, or to start a root shell, use sudo -i

That is because in the other distros when installing them they usually will prompt you to set a root password as well as a user account password. Ubuntu does not ask for the root password during setup, only a user account password of a user that will have sudo access leaving the root account disabled in Ubuntu. If you want to have this enabled, set a password for the root account that will enable the root account allowing for the su command to work.

sudo passwd root

Hope this helps!

By default, su has no password. When you run that command, you're trying to log into the hidden root account on your machine. I'm going to assume that Parrot OS either uses the root account or sets its password to yours, because otherwise it wouldn't work.

If you want to use su, then you can run sudo passwd root and set the password you want to use to log in with su. This will also enable the root user as a full account user, however, so I don't recommend it.

Instead, when you need persistent root privileges in that shell, use sudo su. This will allow you to become root by using your own account and password. sudo -i will have the same effect (and people will recommend you use this instead).