Because the self-licking lollipop of qinetiq has worked so well for us.
The bit that concerns me is the on-paper cost of mil personnel in DE&S and what a GOCO contractor will do with that. They will see mil as an expensive overhead and cut to bare minimum; the result being an even less informed procurement executive on front line ops.
Of course there is some benefit in greater stability/less time away from core duties by removing mil pers, but the effect on the degradation of front line knowledge and credibility of teams with reduced mil experience will be telling.
H