They do - they just never put it together. He's "one weird dude," he's "creepy" around women, he was "moved to the country to keep him out of trouble," he was kicked off the cheerleading squad, he was kicked out of/suddenly left two colleges after rape allegations (though perhaps the allegations were unknown to his friends/family), he was in constant trouble with vehicle violations and warrants in debt, he injures people upon first meeting them, he slings women over his shoulders, he punches strangers in the face, he became subdued/withdrawn - I forget the adjective - after Hannah went missing.
People need to come forward with this type of information, people who know him well. That's what could put this all together. I think they're afraid because 99% of the time he's a great guy, and they don't want to admit that this could, in fact, be he who did this. Unfortunately, while they might think it's loyalty to their friend, it's actually selfish. Withholding information that could help solve a crime. Why? What is the gain?
Hannah's parents are grieving. Can you imagine if it were you?
This type of information could not only help them solve Hannah's disappearance - perhaps even find her alive - but prevent this from ever happening again. If it's not JM, that will be discovered. And these friends would have helped LE figure that out, too, so they can keep looking for the right person (or people).