Could it be because Christian Bruckner doesn't have any history of pedocide or murder?
In fact, he was an abuser, a sex pest and even a rapist as well as a pill peddler, somewhat estranged from the more major crime of murder.
I think the Germans simply invented the Bruckner did it narrative and now they are stuck with it.
No, I think it's mostly because we aren't privy to the BKA evidence to be able to make that determination yet.
Those are not estranged crimes though, as we've already covered in this thread. Studies show that crimes like burglary, violence and rape are the most common indicators found in child abductor-murderers. Having a previous history for murder is actually very uncommon. Only about 1 in 14 people who are convicted of murdering a child following an abduction have had any sort of prior arrest for murder.
Most of the time when children are abducted and murdered by stranger, the event is unplanned. They are victims of opportunity when a sexual predator is presented with a situation they cannot resist. They don't set out with the intention to commit murder, the killing happens afterwards in order to avoid detection.
Even if the Germans are wrong about CB, and I accept they might be, I find the notion they would "invent" such a narrative extremely strange. Why on earth would they do that? What do they stand to gain from such a reckless move? IMO they must firmly believe they have the correct suspect based on what they've found.