Sure, the name recognition is part of it. But let's analyze a bit deeper. How many Americans are on that forum? I'd venture a guess to say a pretty high percentage.
What do we know about Americans? They want their answers like they want their coffee, as fast as possible with no hassle. Many times, when dealing with tutors, students don't want a big hassle. They want to learn, but not have to deal with the "high and mighty" attitude you get from some tutors. These kids don't want to hear a 5 page thesis on the foundations of calculus. They want to make it to college, with a double mocha latte to go. Good morning, Good afternoon, Good night.
Not counting math websites, but face to face contact, the most successful tutors I know are good teachers, but more importantly, they get to the point with their students, and fast. Time spent and results is the general equation. Many times, students may have one of the smartest teachers in the world, but a poor passing rate in their class. A teacher, like a software, or a website, is only as good as the results it produces for the people that use that service.
Case in point: Google. A white page with blue links, yet nobody can touch them in speed and search relevance accuracy. Which goes back to my coffee analogy. | 
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