I can only answer that for myself, not for others. I was a self-professed Apple fan - I preferred iPhones for years because of their style and quality, both hardware and software.
However, because I keep seeing more people with Android phones, I got curious. I ran a small experiment, and purchased an Android phone (Huawei P9) then used it and my iPhone 7 Plus side-by-side for some time, before passing the P9 to my kids and a co-worker to get their opinions.
Style: They look and feel very similar!
Hardware quality - the hardware quality is similar, if not the same. Maybe the iPhone is slightly faster. So, this can’t be a big draw anymore.
Apps - Most apps are now cross-platform, and their functionality is essentially the same. Apps seem to get updated on Android quicker and more frequently than on iOS, due to Apple’s more rigorous approval process.
Built-in functionality - I loved GBoard and split screen mode, and found voice recognition much better on Android.
Usability - I couldn’t see any usability differences, apart from switching between apps being easier on Android. My kids had no trouble using either phone.
Customizability/flexibility - Both operating systems allowed me to customize notifications, data and battery usage at a pretty granular level.
Price - This is the biggest differentiator these days. Maybe people prefer the psychological boost of using a premium-priced product?