A big reason is because HomebrewersAssociation.org login is for AHA members only, while the Forum consists of members and non-members.
When we switched over to our new site 3-ish years ago, we set up a system so a person signing up for membership can create a username/password that works on both the website and the AHA forum. When this type of user logs into the website, they should also be logged into the forum. However, if a person was a Forum user before becoming a member, the two logins likely will not be connected and the user is left with seperate login info.
If you'd like to have the same login for each, give our Member Services team a shout and they'll get you squared away. info(at)brewersassociation.org.