This was done to improve security for the site and user accounts. When someone can pull your username off one of your posts, then they can run software against it to try several thousand passwords that have been collected by nefarious jerks to hack into your account.
We were seeing new posts on very old threads by a user that hadn't been on the site for many years. They were posting spam links obviously with no relationship to leather nor anything useful for members.
It may take you a few keystrokes more to use your email address versus your username, but it has secured your account. People that are logged to this site (with the exception of admins and moderators) can't see your email address. Thus not as easy to hack into your account.
Cyber security has become extremely important. It is critical to protect your private information. This is just one step in that direction. Too much info on the web makes it easier for hackers to get into your accounts, whether it be your bank account, or other social media accounts, eventually leading to your bank, Amazon, or other accounts that have access to your banking and credit card username, passwords, PINs, etc. There are news items about compromised accounts at least once a week, and likely several every day where people have lost thousands $ when their financial information was compromised. Practice safe computing.