What you are seeing is old projects that were dead for years finally rolling over into their graves. A grave lack of new motivated modders joining and existing modders getting demotivated on continueing on whatever it is they are working on.
The communities expectations have soared over the years and consequently, the production cycles on new campaigns has become incredibly long to meet these quality demands. And its hard to remain motivated when it all seems to start to look like *work*. It's also hard to stay motivated when your hard work gets sort of ignored or doesn't get any replies with any kind of substance. Rough gems like The Antagonist seem to be mostly ignored or forgotten (it isnt even listed on the wiki campaign list?) and the technical (and hilarious) masterpiece that is JAD2.21 has a mere 4 pages worth of posts. (I know its hard to have a sensible discussion about the JAD universe and all but come on...)
Motivation comes from many things but I've noticed that HLP is actually a pretty strong place to get demotivated from. Admins and SCP coders that seemingly can't be bothered to get involved in any of the mods. Old members that stopped playing but refuse to move onto the great beyond. And all those things that is funest for a continued healthy modding&player community.
HLP is definitely losing steam, it may not be on the brink of having a cardiac arrest yet but its not exactly looking up right now. Mods in general are slowly becoming a thing of the past, with all the time and effort you need to put into a mod to make yourself stand out, you might as well go indie now a days.