First and foremost, because you want to use iOS. There is no amount of features or security worth using a phone you don't like.
Secondly and on, iOS has regular software updates for all devices at the same time for usually five years until EOL. This year the iPhone 5 gets the drop off. Due to iOS's closed system the security is better and Apple has been implementing even more security features lately, this also allows Apple to have a team of real people checking the code for apps submitted to the App Store to prevent the greater majority of malware. iOS has the largest App Store on the market and a great number of apps are better on iOS than other platforms.
If you have multiple Apple devices, Continuity is a killer feature, being able to switch active tasks between devices in real time. iOS may not have all the cutting edge features, or even some older features that a lot of people like, however, it's a safe bet that when Apple implements those features it will work flawlessly, or at least as expected. TouchID is the fastest and most reliable fingerprint sensor I've ever used. The iPhone 7 Plus also has the most perfect LED screen that any phone will ever see. Apple also has the most accurate on screen keyboard I've ever used. Better than GBoard and better than SwiftKey after learning my touch points.
Really, just play around with iOS in stores or on friends' phones and see if you like it. If you don't like it it's not worth switching.