But it is never shown as a popular topic.
If I understand how popular topics work, it is not the number of replies that triggers it, it is the number of replies within a given time. So if the thread has not been posted in for 1 day (for example) it will not be as "popular" as a thread that is currently receiving 10 new replies every hour.... It may also be that the "like" facility plays a part also.
Bascially speaking, it is about what is popular right now, not most popular ever.
Hope that makes sense.
Also, the "popular topics" system is not able to list topics that are spread out over multiple threads. So if something is "popular", but not being discussed actively in a single thread, then it will not be listed as a "popular topic". Furthermore, if a topic has been around for a while, people are posting less frequently about it than they would if it was a fresh topic.
For an example: This thread about bots
has once or twice been listed under "popular topics", but as it is no longer a fresh issue, many people have already said what they needed to say and thus feel no need to continuously post about it.
We are not trying to hide things or ignore things, so there is no need to remove the tinfoil from the kitchen drawer... not today at least ;-)