Gonna take a couple guesses here.

1. Given that they’re upgrading from a 5 year old phone (that wasn’t the flagship when it was released) that’s still getting iOS updates and that no Android phone has historically had a similar guaranteed amount of support (and currently only the Pixel 8 is rumored to offer more) they might want to be able to keep their next phone for 5+ years.
2. Third party iOS apps are still generally better than third party Android apps and they might value the improved experience.
3. They might not want to deal with manufacturer installed bloatware.
4. They might otherwise be invested in the Apple ecosystem - AirPods, Apple Watch, MacBook, etc.
5. They might want updates ASAP (instead of getting it months or weeks later).
6. They might not want to think so hard about which Android phone to buy.
7. They likely don’t value the advantages Android has over iOS (more customizable, earlier features, actual file system browsing, etc.) as highly as the advantages iOS has.
8. They might not want to learn a new mobile OS, and they might value the consistency and simplicity of iOS.