Oooh. Speculation time!
I postulate that despite their best efforts, T-ara has not been as profitable in Japan (relatively-speaking) as CCM hoped they would be. Think about it, if the profits from Japan were sky high, they would milk that cash cow with every possible gimmick, year in year out. Taking a long hiatus In this idol-saturated market, could easily mean irrecoverable lost ground unless you're a really big household name.
So naturally, T-ara's venturing out to extend and deepen (if possible) their appeal in other markets. China with it's large, youthful population is quite a prize.