Because it's not the price that is at the heart of the debate. FxOS is an attempt at an alternative to the current mobile OS behemoths in iOS and Android and their anti-consumer features and policies.
Edit: and wouldn't it make sense to fork a version of android, like amazon did, to achieve openness and privacy , than to start over ?
For FxOS to really stand out in developing countries it would require a lot more than what it is providing now. In my opinion, since we are partly already doing communications for free trough Facebook and Twitter, something like P2P connections between the FxOS devices does sound doable for me, both for text and voice. In addition, if you could implement something like Twilio for actual calls it would strip down costs both for mainland and international calls. For reference, where I live calls trough Twilio are six to sixteen times cheaper than what my carrier can provide. This way, you would also avoid the need to sign a contract with a carrier. I believe that this form of carrierless phone would appeal to the crowd the FxOS is trying to reach.
1. Does the current admission by the US government of comprehensive NSA spying on the entire internet interest plain consumers? No, it doesn't. "Plain consumers" don't understand the implications nor the technology behind it. So to ask if a "plain consumer" would be interested in FxOS is to miss the point. The "plain consumer" does not have the knowledge to judge the issue and Mozilla is not trying to become a billion dollar corporation to rival Apple or Google.
2. The architecture of FxOS is completely different for a reason so IMO, no, it wouldn't make sense to fork Android.