None of the other major mobile operating systems (open or closed) have the same cachet as Mozilla. In particular, Mozilla has gone to great lengths to zealously cultivate a consistent image of web-centric ideals above pretty much all else. Other mobile operating systems are tainted by corporate interests, even if only in [mis]perception alone when compared against Mozilla.
Mozilla has been trying to reinvent itself and adapt to the mobile onslaught. Increasingly, mobile is being seen as the gateway to billions of people coming online. Mozilla's mission is to "promote openness, innovation and opportunity on the web", and they have seen that desktops are being outsold by mobile units[1] and transitioned accordingly.
FirefoxOS is the foundation's big bet on mobile. Beyond just Mozilla's unique position(ing) in the mobile marketplace, IMO the biggest difference between FirefoxOS and other open source mobile operating systems is that they are betting most of the farm on this. Although the article references that "Firefox OS is a strategic project for Mozilla", I believe that Mozilla's long-term success and relevance will hinge on the success of their Firephones.
Frankly, I'm really happy and amazed that they've been able to get the number of carriers publicly announcing support in this manner that they did for distribution.
Carriers (not necessarily all of the ones listed) don't like how their brands are being potentially diluted and damaged by end-user Android upgrade hell. I don't know what upgrade vision Mozilla sold carriers on, but I sure hope they've got a more solid plan than "we'll copy Android!".
More than likely they said something along the likes of "similar to our near-automated desktop browser update process, less controlling than Apple but not as chaotic as Android." Too bad the carrier meeting minutes and agreements are not listed in the public wiki.
Carriers hate churn -- it's really expensive when they lose a customer. Contract penalties help mitigate economic losses somewhat, but brand loyalty and relevance are getting harder and harder for most operators to retain. Ironically, it's the same desperation to remain relevant that drives the Mozilla Foundation.
Without relevance, mind-share and brand/mission loyalty, it's hard to effect great change amongst billions of people. Money follows when you get the first three factors right.
EDIT: 'Desperation to remain relevant' could be misconstrued as Mozilla being coerced into this particular course of action. I'm not saying that Mozilla is currently in dire straits, but mean that they are voluntarily and pre-emptively putting considerable weight, energy and focus into the mobile arena where they feel they can make a big difference and considerably grow their mindshare footprint, which in turn leads to higher conversion rates for people willing to embrace their mission ideals of web openness.
One of the interesting things of Mozilla's approach is that their entire stack is standard HTML (or, it is planned to get it standardised).
Assuming they stick to this, and other browsers adopt the standard, it means that developing an app for Firefox OS is also developing an app for iOS, Android, WebOS etc. Their big push is for a web-stack, not necessarily for their OS.
