iMessage (and Facetime) and less distractions... that's why I prefer iOS / iPhones.
First and foremost, literally every single person I care about uses an iPhone - between work teams, family, and friends. Whenever I switch to an Android device to mess around with (which is happening more and more seldom b/c of this) it screws everything up with the group messages... the work related ones being the most important. Facetime comes in handy every now and then as well, and it's a helluva lot easier than saying "hey do you have Hangouts?" ... "What's that?" ... So good move Apple, your lock-in is working on me.
Second - and this is strictly a personal problem - I really do love Android. My first smartphone was the Motorola Droid in 2009, and I've used most Android flagships since then. But I'm also a tinkerer. I don't root/rom any more, but even without going that "deep" there's still an endless assortment of options you can do to an Android device. Smartphones are already distracting enough - the last thing I need is 15 other reasons to pick it up and play with it in the few down moments I get. With iOS I get in, get what I need, and get out. I might change my wallpaper every now and then, but that's it. It's less of a "toy" than Android is - for me.