Tom Clancy’s The Division creative director Magnus Jansen took the time out to explain why they did not include splitscreen.
Jansen and the rest of his team at Ubisoft Massive developed Far Cry 3, and it had a splitscreen mode, so Jansen can say that it was not due to a lack of interest or ability on their part.
One reason it does not have splitscreen is because of The Division‘s design. In specific, its progression system requires you will be able to seamlessly switch between single to cooperative multiplayer at any given time. At any time you do this, your character is also constantly logging progress. You want to retain these changes on your own, and without having to worry about staying with any other player to retain that data.
In most splitscreen games, there are also clear cut, defined challenges. These can be as simple as completing a mission. However, for an open world game like the Division, there is fluidity in these challenges. You can freely pick up, abandon and complete challenges. The situation changes constantly, based on who you are playing with and where you are in the game. Having a splitscreen would detract from this experience.
There are also technical reasons for this omission. For example, when players move into Dark Zones, it’s hard to decide how much of the other players you get to see. There are also consideration in terms of keeping profiles online, and graphics rendering.