Hi @raiku, I believe that Hubot-Mattermost currently uses outgoing webhooks to integrate with Mattermost. As a security precaution, outgoing webhooks are not allowed to listen to Private Groups or Direct Messages. We realize this is a bit of an annoyance when trying to integrate but it does prevent someone snooping in on conversations they shouldn't.
We do, however, plan to add a config setting to the System Console that will allow you to have outgoing webhooks listen to any channel. This setting combined with the setting limiting admins to creating webhooks should be secure enough to let Hubot integrate the way you would like without compromising security too much.