It's just general practice. You don't release two games that compete for the same market share next to each other from the same company. Square wants both games to be successful, so they're going to release them apart. The same thing happened with the two games in Japan as well. On average Square releases their games about 3 months apart now from Japanese release to English release, at least as FF games are concerned. Being released at the end of December in Japan makes March roughly the right time for it.
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