Agreed. What if they have enough evidence to suggest (or not rule out) both being in the house at formulated murder time.....may explain that 'last to see' Karen comment the police questioned.......and/or what if cctv captures the possibility of both in car (or an inability to identify who two people are).......and then there was the 'crying/upset' girl witnessed with two men (I think it was?).......
Perhaps, as others have alluded to, this long hold up is more so about clinching evidence to distinguish who did what part of crime with various people knowingly involved/implicated. The need to find more evidence, or waiting for someone to crumble ('have the courage to come forward' comment), may also be linked to allowing the ant to rant more recently......pressure?
IF they have all this circumstantial evidence pointing towards (or being unable to rule out) involvement by various specific people, the evidence is clearly not enough to distinguish beyond reasonable doubt who played which role, and subsequently, hold up may simply be re the inability to lay very particular charges for all those possibly implicated with 100% confidence that is required to be tried in court.