I think the popularity of Paradox was originally built with great games, and they were able to have a large variety in different spanning eras (something you can do historically, but cannot with a future/space title). People playing one jumped into the next, building momentum.
Also, its market penetration has been greatly increased by a large YouTube Let's Play population. You have individuals with over half a million subscribers playing Paradox titles. Distant Worlds is a difficult title for a casual gamer to pick-up, which you could say about the hearts of iron series, but what's more is it *feels* like a difficult game to pickup (for reasons I'm not entirely sure, at least partially the interface). Larry Monte was surely a reason why people had become more interested in this game.
I am immensely excited about an upcoming DW2 release! Small wonder, as I'm still excited by the DW Universe and the patches
Matrix Games on YouTube isn't really a big platform, they should consider getting people like Dastactic to do preview build stuff. Dev diaries only fuel the community who already knows about DW2, but you've gotta spread the word for people to even be able to find dev diaries.