It might not have been possible for an antivirus to detect Flame, because the people behind Flame likely tested it against all antivirus software before deploying it. They were very, very careful. It's how they escaped detection for so many years.

Most malware writers test their malware components against AV software before deploying or selling it.

There are even websites dedicated to testing against multiple engines at once for you, automatically.

Of course, those are collection sites as well...