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I think multiple groups have approached SEGA about it, but so far, they aren't willing to spend the significant cost of localizing it. The last release Dead Souls did miserably (and it really is a good game), and unfortunately fans didn't step up and buy the game--even though it's not the best in the series--to keep the franchise going.
There's a lot of speculation on the costs, but no firm figures. In my opinion, even if they break even, it cuts into the profits they make in Japan on the titles (where they're hugely popular). So not only are they spending money on the localizations, it costs them time and effort that could be spent on the next in the series--which makes them much more money in Japan.