Every platform starts with a small community and with a small base of software, including the Raspberry Pi.
The foundation had several secret weapons to deal with that. The first is that they developed something different. Keep in mind that the hobby platform of the time was the Arduino. They managed to undercut the Arduino in both price as well as performance, and by a quite significant factor for the latter. By addressing the education market, they also fostered goodwill. On top of that, they are active in building and supporting a community.
Other boards may do all of the above, but their attempts aren't compelling. Doubling the speed may attract some people, but it is not compelling when other factors are considered (especially if the price is on-par or higher). Attempts to address the needs of the education market feel hollow, like businesses trying to defend their turf, which does little to generate goodwill. As for community, that is a heck of a lot more than posting videos to a corporate website.
On top of that, the Raspberry Pi gained support from third parties, particularly with relation to education. It has also been particularly strong at presenting their product at conferences (e.g. PyCon's Education Summit). Wolfram shouldn't be discounted either. Given Mathematica's marginal ability to run on the Pi, it was a token gesture. Yet it was also demonstrating support for the Pi as an education platform.