There have been many times this forum was rather empty, but right now it has hit one of the lowest in a very long time. I have been active in this community since Leadwerks 2.2 (about 10 years ago).
There are various events and reasons (of which some were already mentioned) that cause the community activity to diminish.
The largest change was going to Leadwerks 3. We lost several core features: the powerful and ahead of its time real time deferred renderer was not ready at the launch of Leadwerks 3. For a year or so we had to bake our lights. Terrain and vegetation were also gone. It would take several years before it would return. Especially the initial announced price (I think something around 800 dollar scared a large part of the community away). I was very skeptical but I didn't have any large project in development so I just played around with the engine.
Mobile support. I get it that a business has to be run and that Josh needed to inovate. Josh tried really hard to make it happen but it was not meant to be. Many people didn't like the mobile direction since Leadwerks's strongpoint was its renderer.
Flowgraph: This is something I never understood. Flowgraph was there from the start of Leadwerks 3. Since then, nothing has happened to it. Why? The core mechanic is there and it works. I remember that Rick and I made 2 little scripts in the first days of its release and they were so much fun to play with. But without zooming, layering and grouping, the flowgraph is just an empty tool that is pretty much useless for an actual game. For a small tutorial or demo scene, it it useful, but after that it becomes unmanageable.
The website, forum and documentation have had soo many changes during the last years. At some point it looked like this was the only thing that was in development. Of course maintaining and having a proper website is necessary to a certain level, because it attracts new customers, but it really felt like this was taking a lot of time of engine development. That said: The current forum, with documentation and store it the best version yet.
Steam: Moving the selling of Leadwerks to Steam was a needed step. Not everyone liked it (I personally did like it) as they wanted to have a separate installer rather requiring Steam for the editor to run. I think it made spreading the software easier. Especially with beta branches, features could be tested more frequent.
Steam workshop: From my own experience: just plain awful. This has nothing to do with Leadwerks in my opinion. The Steam interface was (and still is) outdated in design, sluggish and lacks to many common features for it to be attractive. Luckily this is replaced now with the forum's integrated web shop which already works 10 times better than Steam.
Game launcher: Conceptually really great but for me it never really took of. Often it didn't respond and without curating any games, it scared more people away than that it actually attracted.
Default Lua editor: the highest level of frustration of working with Leadwerks came from using the embedded lua editor. For beginners it already didn't help that there was no auto-completion (this was added later). Proper documentation popups like most mondern IDE's do was not possible. If you compare that how other default tools do that, then you are going to have a much more difficult time as a starter. Luckily Visual Studio Code is going to change that.
Debugging: maybe that is just me, but debugging always felt like a pain. I would end up iterating over some weird table over and over again or I would hit breakpoints that came out of nowhere. Again, especially for beginners this is a real demotivating factor.
Editor customisabilty: Again something personal: the biggest issue that I think causes people to go away is the editor customisability. No assigning of custom hotkeys. Not being able to add shortcuts to toolbars ("Create pivot" for instance). No multiscreen support or customizing the editor layout. No custom editor plugins or tools support. How many tools haven't I made in Unity and unreal, that extend my workflow. From simple drawing lines to see where my characters waypoints are to generating structures in the scene tree like ropes. Maybe performance may be an issue in those engines, but the workflow for creating gameplay and level design is so much faster because of it.
Documentation: @gamecreator already said it: there is never enough documentation. Ironically the documentation website looks pretty good in its current state, but really needs some updating. In the private Turbo section of the forum, I hoped to convince Josh to move the entire documentation (including API) to github so that the community can help out. With Turbo I would really like to help out were I can making new tutorials, templates and what not.
Many of the issues above are going to be solved with Turbo. Josh's blog for the last few months have been a joy to read. Josh figuring out all these insanely crazy and complex technological things is amazing. I certainly couldn't do it.