Because the case is not strong enough with just the circumstantial.
I think a lot of people fail to understand what "circumstantial" evidence actually means. They hear the word and think it means it is tenuous, weak or flimsy evidence. Most convictions are based on circumstantial evidence, the alternative (direct evidence) is not that common. Direct evidence is something that directly implicates the suspect carried out the crime. So a video recording of the crime for example, or a witness who saw the crime take place would be direct evidence.
Forensics and DNA are usually classed as circumstantial evidence since they don't directly prove the suspect carried out the crime, only that they were there (at some point in time). I know HCW has stated they don't have any scientific (forensic or DNA) evidence either but just pointing out that most evidence used in cases is circumstantial. Even if an eye-witness saw CB bundling MM into his car on that night, it would still only be classified as circumstantial evidence that he murdered her (even though it would be direct evidence he abducted her). Or if they had a photo of them together, this would also be circumstantial since it doesn't directly prove he killed her.
The point is you cannot determine that the evidence is "weak" on the basis that it is "only" circumstantial. People are using the word as if it means something that it doesn't.