Because of the anniversary and also because it is used for marketing

Because it's easier with Roman numerals

Because it is the tenth anniversary (if I am not wrong). The number 10 has been replaced by the Roman X.

It's because it's the 10th anniversary (if I'm not mistaken).

Marketing, people wouldn't buy an iPhone that's called 10, instead the X sounds professional, more expensive and so on

Hello To answer your question

iPhone X honours the tenth anniversary of Apple's flagship phone, so that X should read 10. So we should call it iPhone Diez and not iPhone Equis. Also, let's remember that in many Spanish-speaking countries, the word equis is also used to describe something of average -- or mediocre -- quality.