It's true, we don't know what the usage would have been like if we did some real marketing of the Metro feature. We briefly had a "what's new" page that promoted Metro for users of Firefox Aurora on Windows 8, but we never did a similar promotion on the larger Firefox Beta channel. We had another in-product promotion planned for after the release of Firefox 28, but that's no longer in the cards.
In the absence of that, we have to rely on metrics from desktop Firefox (e.g. what portion of our users are running it on the touch hardware that our Metro app was designed for) and on any data we can get about the PC industry as a whole.
I'm torn about this decision (understandably, I think), and I still think that Microsoft has a good chance of eventually building a much larger user base for its touch platforms. But the improvements we wanted to make to the Metro browser (like making the scrolling as smooth as possible) would have required ongoing work not just from my team but from other groups like the graphics and layout teams, whose energy may be better used on other platforms (desktop, Android, Firefox OS). So we had to think strategically about what's right for us to ship this year.