Why place 1 order if you dont receive it together

So I placed a order for 3 items last week, all of it on 1 order, because I wanted the delivery to be together as these items are for my daughters. Yesterday on a sunday the courier company arrives at my house with only half of the order, when asked when I will get the rest of the order I was informed that they will only come here again on saturday or sunday. Why did I order everything at once of it gets delivered a week apart. This is unacceptable. This caused big upset for my youngest.