I bet it has to do with the average age of the gaming community getting older. I used to play Civ 5, EU4, CK2 all the time in college, when I have tons of free time and didnt care if I was up until 3 in the morning. Now that I have a life and a job, it takes like a week of 1-2 hour sessions to finish a game of civ 5, and the last time I played EU4, I played for several weeks and didnt even finish.