Here is my guesses:
* iMessage. Teenagers are pretty social and try hard to fit in. Teens may feel more "cool" and "in" to use the same type of device, and iMessage also cements that.
* Affordable. iPhone SE is $400 which is still expensive, many parents may see that as affordable for their teen
* Parental controls. Apple has a staggering number of parental controls, does Android match this in terms of features and ease of use?