Because it's a cute little piece of Star Wars porn, whose story doesn't matter one way or the other and which even wipes its slate clean at the end. So you can watch it, enjoy it and then safely forget about it.

The main point of SW contention is how well non-OT material relates to the revered OT. The prequel trilogy, for example, undermines OT characters and themes (Vader/Boba Fett and midichlorians) while the sequel trilogy messes up OT story and character development (RotJ ending invalidated, Luke Skywalker). The movies have other problems but these are considered some of the worst offenders because they actively tamper with, and might devalue, some people's idea of "perfection".

Love or hate Rogue One, at the end of the day it is a uniquely harmless movie in the Star Wars Universe. So why even bother having an argument.