Most likely because there is no-one following your category. If you turn on the site setting activity_pub_verbose_logging and look in your logs you will probably see a message that looks like this
I’m definitely going to put a “reason” in that modal as this has come up a few times now! Follow your category on Mastodon, and then create a new topic.
**edit or perhaps I will just change it to “published” in that scenario, as it does in fact add the activity to the outbox. It just doesn’t send it anywhere (as it has nowhere to send it to). So it technically is “published” (i.e. it’s in the outbox).