Different people, different taste and different status etc. Some buy the iPhone just because of they have huge money and have to spend at any cost on the best brand, they chooses just as a top brand for any gadget, some don't have money but still value it and try to buy it in any option like EMI or other way. Some people buy it just to give as a gift etc.
iPhone generally draw the attention as 'the top' one, so many would like to grab it.
Now coming back to iPhone device performances (not all but stating few), in fact, no other brand can chase its touch-integrated feature and security level at the moment. It has its own iOS whereas other brand devices all are depending on other's OS.
iPhone Camera may be lesser MP at the moment compare to the other brand but try to take the picture or video from the latest or oldest iPhone and upload them in a system and check it, now do the same from other brands' higher MP photos and videos upload them in a system, check and compare between them. You will feel and realize which one is really better.