The #1 reason is Apple's focus on user privacy and the device's purpose to serve the user through of specific set of features (note I do not mean they give us control of the features, just that they designed it for specific purposes). Apple's business model does not rely on monetizing user data and it shows in its products.

At the time I switched (Nexus 5) there were abundant issues with the Android OS from a memory leak issue that required rebooting the phone periodically to design flaws such as Android's UI rendering de-prioritization. Much lower quality of apps in general. Much worse customer service experience (you can't just walk into an Apple store and get an issue fixed). Much worse hardware quality (part of the reason I switched back was because the Nexus 5 charging port stopped working and started emitting smoke. For a while i was charging purely through the Qi charger. I've never had an iPhone break on me, and any other issues were always resolved for free at the Apple store) Not sure if these are fixed, but I cannot switch back as it's basically impossible to use Android without Google services and consolidating all my data with Google. If google wants me to give me data they're going to have to pay me more.