Board Thread:Theories and Speculation/@comment-24824859-20150212011837/@comment-1512913-20150213162609

Because they can't use it. My deduction is that chakra mode is a part of the whole convert chakra thingy Naruto does before chakra transfer. Chakra mode is the same just on himself. Kurama's chakra is taken and he makes it fits his own chakra signature, boom Kurama's chakra turns yellow and gets his chakra signature - hence chakra mode.