The reason purportedly has nothing to do with the oft-cited "Madden curse," but that Tomlinson was not happy with EA's financial offer. Electronic Arts, according to Rovell, understands their cover to be what he calls the "Wheaties box endorsement," which means that EA will offer the cover athlete less than his market value. Tomlinson opted against it.
Young, afraid of no curse, is now the cover athlete. But as Rovell justly observes, Madden titles sell well regardless of who graces the cover.