I would wager that most users would not recognize a “banner topic” as they would a topic that lives within the feed. In fact, it’s not really a “topic” at all, it’s more of a notification bar. It feels … like a promotional tactic.
Communicating something to the community through the feed takes on a level of value that the banner option simply doesn’t provide.
Finally, when you’re communicating as an admin within the feed (especially on a site in which that admin is known and respected), there’s a certain amount of weight you carry site-wide—that is all lost in the banner topic.