Fox has never come out and "officially explained" why it got cancelled. But, in general, networks don't do that because they don't need to.
With extremely rare exceptions, shows get cancelled for exactly one reason: bad ratings. That means either:
- The show's ratings don't justify high enough ad revenue to cover costs of production, or
- The show's ratings are worse than most of the other shows on the same network.
In this case, probably because she got asked about it all the time, Gail Berman has briefly discussed the issue. It wasn't an official statement, but as Fox Entertainment President at the time, it would have been her call. She says:
"Canceling 'Firefly' was as difficult as anything I'd ever been involved in because Joss and I had been creative partners at one time [...] I worked with him very closely on this particular show and when it didn't perform [in the ratings], having to cancel it was very difficult." src
As far as why it got bad ratings, as you mention, fans have plenty of theories, mostly ones that make Fox sound bad. There's likely some truth to some of those, but it's also important to note that, at least back when Firefly was airing, "genre" shows did not do well compared to "safer" ones. That's a trend that has started to change recently, but Firefly is in some good company.