The press corp shoots with what they are given in most cases. During tight economic times, newspapers (or corporate owners) don't have huge budgets to replace hundreds of thousands, if not millions of dollars, in lenses etc. Nikon was the first on the scene with affordable DSLR (original D1) and the papers snapped them up. Now they are stuck with slower performing, lower resolution bodies.
However, when you view any major US sporting event, it's usually a sea of white (Canon), super telephotos lenses and an occasional black tele belonging to the few remaining Nikon shooters. The superior Canon telephotos (all IS) out perform Nikon's long lenses. For several years an ever increasing number of pros have switched to Canon for the superior optics and more varied line of AF lenses.