C'mon, guys. We all love Nikon, but it doesn't take much to determine that Canon dominates in the photojournalism sphere. As was mentioned, just look for the white lenses. Given that my brother is one of those folks with the white lenses, I might have some insight.
The chief reason is likely that most of the folks working for news agencies and papers full time shoot with cameras owned by their employers. My brother's paper is owned by the Newhouse family, they own a bunch of papers, and they all shoot Canon. Regardless of what the photographer may prefer, when you're handed a camera you use it.
Second? From all I hear, Canon's Professional Services is incredibly good. Not saying Nikon's isn't, but all you need to do is watch the Scott Kelby video where he talks about them sending him a camera to use and you'll get a feeling what they do - and they don't just do it to the guys like Kelby. When my brother has a lens go down, one call to "his guy" and he's got a date for lunch the next day and a replacement. At a big event like an NFL game? The guy is there.
Third, photojournalists shoot jpeg. DxOMark will tell you every day to Sunday that Nikon has the better sensor, as will other sources. But there is much evidence that the noise reduction and other in-camera functions applied to jpegs, and not RAW files, is a cut above Nikon's - another thing echoed in the Kelby video.
Would I worry about it? Only if I had a lot invested in Nikon and got hired by someone and couldn't "share" gear.