IO's Next Hitman game will release episodically
Hitman still technically releases on the same day, but at about one-third the size. Only two missions—the Prologue and one set in Paris—will make the March release. The other two that were planned for the initial release, set in Italy and Morocco, will now follow in April and May respectively.
It seems like the team simply ran out of time and feels like they need to ship something sooner than later, which is disappointing. It's either that, or they were simply told by the people in charge that this is how it had to be. Personally, I'd rather they just took whatever time they needed to roll the game out all at once.