My guesses on why CK2 is lagging behind:
1.) Like what many of you said, CK2 is growing long in the tooth. The graphics aren't as flashy as EUIV, HOI4, and certainly Stellaris. As well as being old, in comparison to its GSG brothers, it also looks old. This in turn has new strategy gamers looking at something more attractive.
2.) Being the first game in Paradox's now universal DLC policy, it has accumulated tons of DLC. I know there's a lot of debate about whether the DLC policy decreases the quality of Paradox games over time, (In fact, I wholeheartedly disagree, but that's a story for another day.) but when a possible new buyer of a grand strategy title goes to Steam and sees Stellaris with 3 pieces of DLC, and CK2 with a whopping 26 pieces of DLC (including Jade Dragon), it's not hard to see which title is less daunting.
3.) Paradox is significantly better at making games that are more intuitive to learn than they were in 2012. Pretty simple.
4.) This varies from person to person, but I know many people who are a fan of Paradox GSGs but can't get into CK2. If you're a CK2 veteran, it may seem a little perplexing, but someone who came from Total War, AoE, or even another Paradox game may be caught off guard by CK. This is due to the character-focus nature of the game. It requires a sort of paradigm shift for a strategy gamer who is used to playing as a nation, or entire faction. Like, Victoria 2 is an incredibly complex game, but at least you still play a nation.
Anyways, those are my stabs at why the CK2 situation is what it is. Moving forward, Paradox should keep doing what they have been doing with HOI4 and Stellaris: create intuitive games with a DLC policy that places emphasis on more meaningful, larger pieces of content, rather than DLCs like Mare Nostrum and Sons of Abraham.