That's a very good article, which goes into quite a lot of depth.
But in a nutshell, everyone in the US does it because Bud did it.
I seriously doubt most guys in the US (watch now as Willmer, Manny et al leap in to prove me wrong!
) can even give a coherent reason to use the C trumpet rather than the Bb.
The article Govtmodel linked suggests that the C is less prone to splitting notes, but frankly I think most people would agree that it's the player that splits a not note the horn. The only plus side I can see is that transposition becomes marginally easier, Bud mentioned this in something I read about his motivation for using the C trumpet.
Apart from that, I think it's just a sheep mentality, young Americans get into thinking that they
need a C trumpet to play orchestral music with the big boys just because all the top guys in the US orchestras now use C trumpets for most repertoire.
Silly really.