Differentiating themselves within the market isn't too far from the list of reasons, I'd suspect. Who would be reading about this if it were just another Android mobile phone?

This is targeting a small market of Linux enthusiasts with a different kind of operating system. If this were running Android, why wouldn't those enthusiasts just buy one of the market-leading phones and use a well-supported ROM?

I think that they are aiming at using a mainline Linux kernel to make it resistant to the prevailing Android problem of lackluster updates.

Running mainline Linux is way harder as it requires decent support most android devices do not have.

Linux almost implies Android will also work. It'll surely be an option.

Because then it would be a yet another completely unremarkable and uninteresting device with no reason to buy it, like other Android or iOS phones.