Of those four titles listed, Fruits Basket is the only manga not published in the USA by Viz Media...and Fruits Basket was a thing of the previous decade, so Viz pretty much had this in the bag. (Of course, the big difference here is that Fruits Basket ended a long time ago and the franchise fizzled out by now, whereas Pokémon is ongoing.)
Someone suggested D.N.Angel here...which would be a realy weird choice for a KIDS' contest. D.N.Angel, while shounen, is aimed an older age group than Nickelodeon, whereas the four titles here would appeal very well.
I would've bet on Naruto though. One Piece may be intensely popular in Japan, but it failed to get a foothold in the American market. Quite a shame too, as it's my favorite of the four. Naruto on the other hand, consistently gets multiple slots on the top 5 best-selling manga at Borders and at Amazon. My best guess as to how Pokémon took the spot from Naruto, despite the difference in popularity of the manga, is that the Naruto-reading crowd tends not to watch Nickelodeon. There are a lot of Japanophiles in the Naruto fanbase, and Nickelodeon's programming is as American as you can get. (Avatar doesn't count because it ended its run.)