1 The phone's dimensions make it brittle
The big size of the iPhone makes it easier for the screen to break. For the phone to be stronger either the sleek design must change or the screen size must become smaller. Both of those parameters go against the basic design of apple phones.
2 The screen breaks easily
The screen of the iPhone is the weakest point of the phone. The large screen increases the probability that the screen gets broken when the phone falls.
3 The thin design makes it brittle
Customers want thinner and lighter phones. To meet those demands the strength of the phone had to be sacrificed a little more. This resulted in phones with brittle screens.
4 The rear camera is exposed
In iPhone 6 the rear camera is exposed in such a way that a drop could damage it easily. Because of the camera's design a part it protrudes thus making it easier to break. (See Why is the iPhone so expensive?)
5 The phone is hard but still brittle
When a phone becomes more scratch resistant it usually becomes more brittle. iPhone screens are designed to be scratch resistant but this results in more brittle screen.
6 The corners aren't protected
Unlike many other phones that have corners that are protected with hard metals the iPhone's corners aren't well protected. This sometimes results in shattering of the screen even when the phone falls on one of its corners.
7 Protection isn't used
iPhones are best used with strong covers and screen protections. With the absence of protection the phone can easily break if it fell. (See Why some people think the iPhone is the best phone?)
8 It falls easily
Some people suggest that the larger size of the phone makes it more likely to fall as people fail to handle it properly.
9 The screen is very large
The fact that the screen is very large makes it more likely for the phone to fall on the screen when it falls. This increases the chance of shattering.