First of all, thank you for that weekly updated comparison,
@IsaacCAT !

I'm looking forward to it each week...aside from simply liking diagrams, it is interesting to follow the player counts over time.
You likely have a point here, though I would add another, purely game-mechanic related one:
Strategy games (be it 4X or GSG...and likely most other sub genres as well) are always subject to an inherent snowball effect. It is their DNA. Take it out and you conflict with the principle of a good decision having good effects helping with you in future when the next decision comes. As a developer you can and have to limit it, but you can't escape it. And anything related to warfare is the snowball effect on steroids: You invest and if you win, you not only gain X (which you might get with peaceful buildering as well), but also an "Y" by damaging the opponent (taking land, destroying stuff or causing whatever cost warfare creates in any game). The overall process is also called map-painting and it is something any game has to deal with. And here HoI4 has a huge strategic advantage: It can technically celebrate that "snowball painting the map" as long as the loose historic boundaries it tries to follow permit it, but no one will ever critize the game for being "war centered", "war being the (only) way to play the game" or "map painting being the goal" (ok, if you RP, the latter is not realistic for every nation). So HoI4 can solely focus on getting the math&mechanics behind warfare right and sparsely sprinkle in other stuff (ironically even up to experiments in the direction of allowing to become an arms trading tycoon in the upcoming AAT expansion via the new International Market feature)
Every other Paradox GSG OTOH has the challenge to keep warfare as an element within other features, topic and approaches, so it should not become too powerful and dominating - and while that goal is pursued with all the devs effort and varying success, IMO it can never be perfectly achieved...which is not the devs faults, but caused by the the initially meantioned snowballish nation of the games and the fact that war is being the fastest way to enlarge your snowball. So while every other game constantly needs to pull the breaks here, HoI4 can give full speed - and maybe the dynamic this absolute coherence between the main topic and the main gameplay mechanic creates attracts people. You are in for strategic action, you get it - and you don't have to regret anything because of the absent feeling of "Hmm, shouldn't [Insert any other Paradox GSG except HoI4] reward stuff aside from war more?"