Indeed, they've toyed with all manner of unique flavors for their KitKats. According to this article, wasabi and melon flavors , among others, have made their presence in the bars, so a pink, fruity chocolate like ruby would be an ideal fit for the Japanese market.
That may be why it is debuting here, rather than in the US, where consumers are very passionate about KitKats and may be less receptive to anyone messing with the tried-and-true favorite. If wasabi-flavored KitKats can work, wasabi-flavored bars are probably a shoe-in. The mass market in the US may not welcome neither. Time will tell, and I'm sure the ruby KitKats will find their way here eventually.
A delayed US debut may be a marketing strategy: Get a buzz going overseas about a new version of one of America's favorite candy bars that someone else has and they don't. Time will tell.