I would love to read some surveys on this matter, because I highly doubt that security/privacy is a significant factor for more than a few percent points of iPhone buyers.

(I base this hypothesis on the fact that, in virtually every field of technology, end users have shown that they will overwhelmingly sacrifice privacy and security in exchange for convenience and cost savings.)

I expect that the main motivations for iPhone purchases are aesthetics, performance, ease of use, integration with the Apple ecosystem, and especially fashion/status symbol factor.

As for the rest of the comment, as other people have already pointed out, you can prevent evil maid attacks by putting the legally-required "open" mode behind a factory reset. Which is roughly what Android phones do with their unlockable bootloaders.