I think it's simply because the Big-5 is apparently based on science whereas the MBTI grows out of Jung and metaphysics and thus even though the results support the unscientific theories "scientists" write against it. I have taken a Big-5 test and it pretty much correlated with my MBTI, including the spectrum nature of each pole. The only label difference was the Big-5 had me as an extravert while MB has me as an INTP and thus introvert on that pole, and I think that the Big-5 labels me E because I am a not shy introvert.