Phoenix appeared at a time when web browsers were bloated, and it was refreshing to have a minimalist app that simply browsed the web. It wasn't that potential was being stifled; other browsers had too many features. It was that we could download a small binary on dial-up, that opened up quickly and just did what we needed without any fluff.

Notice that last part about downloading? That's the key. We could download Phoenix, and run it. I can't download FirefoxOS, and run it. My devices are locked to what they can run. If mobile appliances were like personal computers, then a Phoenix could rise. But it's a completely different situation, so there is no way to compete like you could with web browsers.