Think about the historical origins and current leadership of aikido for a minute. What do you think the reason is?
Case in point, albeit from judo but no less telling:
Note that Ms. Fukuda, the last surviving student of Jigoro Kano, was awarded her 10th dan rank not by the (Japanese) Kodokan, but by an American judo association. Her male contemporaries in Japan had long before received both their Kodokan 9th dan (Ms. Fukuda did not receive hers from the Kodokan until 2006) and their 10th dan ranks. I wonder whether the Kodokan will relent and award her the 10th dan. She will be 99 in a few months!