Because they are both startups and they both get coverage from the same tech news media. Which makes the inequality of coverage relevant given the amount of users.
Let me give an example to try to explain it better.
People always complain that Celebrities like Lindsay Lohan get far too much coverage by the mainstream media while important Government issues like the Stimulus package get too little coverage. The media's excuse is always "we're delivering what the public wants to hear." But if someone could somehow prove the public wants to hear about the stimulus package more than Lindsay Lohan it would make for a great counter argument against that menatality.
That's what this comparision is (with Twitter being Lindsay Lohan and StumbleUpon being the Stimulus Pacakge)