Here is another very overtly critical answer to your question from a passionate iNatter who is occasionally on the forum.
Maybe there is not much of value to gain on the forum. It is mostly a place where users solve problems that you do not experience, or people have general questions about a topic that is not necessarily of your interest. So why bother replying to that? Let the admins fix the problems and people who like a certain topic reply to it. On posts that do interest you, other more actively forum-addicted users may have already commented something of value, and so there is no need to regurgitate what has already been said just for the sake of being active on the forum.
One of the most annoying things of the forum is also that pretty much everything has already been posted once and that therefore as a reply you will often receive a referral link to an older topic, sometimes from way back and with fifty or more reactions. Good luck reading all of that and it is not the kind of enthusiasm that you were hoping for. An interested user asking “What was your most recent lifer?” will be referred back to a post from 2021 with the same question and we then wish them good luck reading all of the 400+ answers from that glorious November 2022 when some other user commented “Chestnut-marked Skipper” or something. And when someone asks for an ID, they are referred back to iNat itself, of course.
So for the majority of users, what is the value here? The forum is paradoxically either too specific or too general to spark an interest in participation for most users. (And what’s the value of earning badges for being active on the forum? A kind of childish incentive to motivate people to be active here.)
Also, the design of the forum may be somewhat boring and chaotic. It’s just a few tabs with categories and endless lines of replies that are chaotically organized. Lastly, maybe being visible to the public on the whole and not knowing who and how many people will read what you write (with my somewhat negatively interpretable reply as a great example), may deter people from participating here.
All of this being written, I deeply love iNaturalist, its interface, how iNaturalist itself is designed, the community on the whole, the interaction with other users through observations and instant messaging, etc. The forum is just not what I am interested in per se. It is just an extra tool to sporadically use if needed. In short, I guess the answer to the question basically lies within the question itself: most passionate iNaturalist users are passionate about iNaturalist itself and that is simply why they are not active on the forum. My two cents.