First of all, it's the design. Most of their products have all-around great specifications for what they are. Obviously they're expensive, but something like an iPhone have attention to detail to everything. Not like most Android phones. They might have great camera but awful battery life (Galaxy S6, for example).
Secondly, they have hands down, the best integration of all of their products. If you have an iPhone you can write SMS, answer calls on your MacBook Air. You might say you can install Pushbullet, and people like us, will do it, but the majority of people just want stuff to work and Apple delivers that greatly.