You are looking for the
From the man page:
The -H (HOME) option requests that the security policy set the HOME environment variable to the home directory of the target user (root by default) as specified by the password database. Depending on the policy, this may be the default behavior.
In your comment you specified that you want to know why this happens. The answer is that
sudo has many compile-time options. Since you did not specify which distribution you are running, I'm going to assume your distribution has set the option to preserve the
$HOME environment variable.