I believe it's either because of this or whenever something is free people gets excited. They want it. They get it and due to almost zero effort while doing this, they lose interest almost immediately. (See my free steam games example above.)
What you're offering and doing is great, I do truly support it and appreciate it. You've been clear and left zero question marks for me. I have another suggestion. Add a filter that will terminate 90, 180, 365(?) days in-active forums.
You may try to encourage users for some actions. Such as:
Instead of "Create your own Flarum forum instantly.", "Create your amazing new community, instantly!", "It's like magic, you'll love it!" and such. This way people will gain an initial boost stat to their interest. (+5? ?)
Of course those are just my ideas, could help or not. Whatever is the outcome, best of luck.
