because it is much cheaper for them to produce, so more profits... second thing is supply issues , they cannot manufacture enough exynos cpu to power up global launches..
they started this trend because of LTE . exynos didn't had 4g LTE at that time , so samsung was forced to go with qualcomm for US/uk markets for LTE .. maybe it worked out great for them profits wise , so i think they don't want to take any chances by breaking the winning stratergy