It’s Google’s choice. Propably a topic is easier for users, or googlebot sees it that way. Or that forum link gets more clicks. Or your WordPress is offering too much all kind of popups or another junk, or is way slower.
So actually you should solve out possible issues on WordPress end. Because basically you are askin now how can I make a topic look worse in eyes of googlebot than WordPress 
Canonical url doesn’t mean that Google ranks original one better. That happens quite often though, but it is not automatic. It basically tells Google that those aren’t dublicates but connected copies, like mirrored ones, and for search results those two are same. And depending what is searched, who is seaching and perhaps which one offers better meta, Google chooses one of those two.
For me that started happen when I dumped oneboxes and started show full copy.