But it depends on how you look at the content. The entire Coil was one big area (you end up in the same room once you finish each of them) made of three big corridors leading to the center where you get to fight the final boss once you cleared all three corridors bosses. In the end it ends up as how they come out. Do you want the dev team to give you everything at the same time and then nothing for a year? Or do you prefer to cut it in parts and have multiple entry points that are basically just "save points" but you get a new one every 3 months?

another point is accessibility. Like someone mentioned, doing a raid that is too big and takes too much time to complete will keep people away from it. You could find a way around it by saving your progress but then you can't farm a specific boss if you want to. The way FFXIV handles it makes sense from that point of view, coil and Alexander are one big raid each with several rooms and bosses that are connected. But you choose which one you want to do when they are on df and have a saved progression when it's freshly released. That way everyone can do it at his own pace.