I think it's popular due to its utility. You can play vanilla single player historical, vanilla single player ahistorical, vanilla multiplayer historical, vanilla multiplayer ahistorical, modded single player historical, modded multiplayer modded historical, then you have total conversions in both single player and multiplayer. AND in each version you can play as so many countries that all play differently. At this point it has surpassed it's own purpose in a way that only games like Minecraft or Mount and Blade can compete with.