The primary reason for their disbanding is that drummer Yusuke is suffering from spinal stenosis, which causes pain, numbness and loss of motor control, so he cannot continue with GHOSE to the best of his ability. He originally wanted to leave the band but after several discussions GHOSE felt it would be best to disband rather than replace Yusuke. He will, however, be playing with the band through their last live despite his condition.
I'm sure there are other reasons but that's what most of the member comments on the DCS homepage refer to: http://dualcore-sound.com/index_pc.html.
Jujo joining was long overdue, but too bad it's not meant to be for long.