It would just been unnecessary overhead to be honest. Remember everything that is encrypted needs to be decrypted and depending on the complexity of the encryption algorithm being used, it could end up causing more problems than it would likely solve. I often forget my usernames for various services so being able to be sent my username without having to reset my password or anything is very helpful.
A properly built web application should throttle dictionary attacks out-of-the-box and safeguard against these kinds of common username and common password list attacks which are very inefficient. Plus, if the attack is originating via a login form you're not really protecting anyone because the username and password are being sent as plaintext to the server and would be encrypted anyway. Throttling and banning rapid login attempts is the only way to efficiently go in this instance.
The only benefit I could see is if someone were to gain access to your database and dump out all of the rows of your users table, then encrypted usernames might make sense (but still not in a way) because you'd most likely have identifiable unencrypted information still in the database like names, locations and email addresses. If you were to encrypt everything, you're crazy.
You would then run into problems if you were running a forum for example and wanted to show the usernames of users online or the username of someone who authored a thread. Think about it, you'd be decrypting that username and then caching it somewhere in plaintext anyway (unless you fancy straight-up repeated database calls for the same information over and over again).