In the internet-landscape, Mozilla is the one that tries to intervene on the user's behalf against commercial interests. Maybe they thought they could do the same for the much more competitive mobile market.
The bigger problem i think is that Mozilla didn't make a phone that appealed to influencers - the high tech crowd similar to the early adopters for Firefox. Instead they tried to go for the emerging markets that was already well served by other alternatives, and who aren't ideal early adopters. From what i saw the user experience and quality of their Platform wasn't top notch either (and html apps still suck in 2015).
Ubuntu was on the right track with Ubuntu Edge (the Indiegogo campaign showed that there was definitely a demand for it), but they set their goals too high and since then they seem to be dragging their feet in getting into the market.