As no doubt the situation with the MM inquiry - the requirement for admissible evidence is paramount.Could be, along with what he sees as back-up 'evidence' from CB's 'Buchs' which seem to form some part of the evidence held by the BKA. It seems a very flimsy and threadbare premise for the abduction & murder claims made though.
Be interesting to see where the two unknown victims/unknown dates on the current charge sheet go. Nothing but word of mouth from a pair of criminal associates of CB, but they've nevertheless obviously met enough potentially chargeable criteria for a judge to give the go-ahead to take them to trial.
Prosecutors and police take great pains to get the best evidence possible and they have obviously been very successful in doing so in the horrible rape cases you mention as well as DM's aggravated torture and rape for which CB was convicted in 2019.
Evidence gathering is no easy task when as in MM's disappearance there is such a gap between the crime and the punishment.
Fortunately for the sake of justice the German investigators seem to be particularly adept at fulfilling the task.
My opinion