It's a good question Kiyoshi.
I think part of the answer is that even for solo playing a connection to the servers were required.
Now is it an issue on blizzard's end having not planned enough servers (at least authentification servers), only them know I guess.
I'd like to see some of their communication on the issue though.
Edit: Apparently few and unexplicative communication. I wonder if we'll ever know what really happened.