I imagine that there was a disaster in which Eva and Lori died, and Jazz hated being a king anymore, so he joined R.A.B.T. to forget about the past. Dark Shell and Devan Shell were the ones responsible for that, so Jazz attempted to destroy them once and for all. But Dark Shell manged to survive... It's just my wild imagination. Probably the truth is that nobody from Jaleco ever tried to know the story of previous JJs.