What is your main reason for thinking he’s probably not BG?
My gut or women's intuition. Many long-time posters in this thread are familiar with my personal story. My eight year old daughter was abducted and murdered over 20 years ago. Her case was "solved" earlier this year. I took a six month vacation from WS and social media because reading the cases I follow became too difficult.
Like Abby and Libby, my daughter was found in a rural area. When the crime scene is in the outdoors, the area becomes very large and difficult to investigate. Especially when there are volunteer searchers involved. Spit, urine, snot, gum wrappers, pieces of paper, leaves, sticks/branches, grass, etc. must all be collected. Finding the DNA of the person responsible is not always an easy task. This is one reason LE is not always quick to confirm someone is a suspect. It's entirely possible that DNA found at a crime scene is not "pure" (mixed with other bodily fluid, touched by someone not involved, touch DNA, a small amount, etc.).
Sometimes you have to read between the lines to understand what LE says publicly. My belief is that LE has a very short list of suspects. The DNA collected may be explained away (volunteer searcher, LE, medical personnel, family, etc). This puts LE in a position where they have no way of making an arrest without a tip or confession. They need proof that the POI was at MHB on February 13th. They need someone to reveal they are providing a false alibi. They need someone who noticed something unusual about a person they know after the murders. They need someone to provide that one tip they have been waiting for. I can almost guarantee you that JBC has already been interviewed about Abby and Libby since his arrest. His DNA has already been collected. LE has some way to rule a suspect out (think DN, PE , RL, etc.), we just don't know by what method.
Again; I would love to be wrong. This case is very near and dear to me. My heart just tells me it's not him.