I think it came down to actually proving "diminished capacity".
Also, I will always wonder why the (still unknown) witness who saw JC and her children that morning let them leave his/her home and go out into the freezing temperatures when admittedly they weren't prepared.
I think this case will never fully be resolved for me including your question. I would still really like to know who that witness is and what exactly friends and family said in interviews. It still doesn't make sense, but I am happy that the grand jury was held and they heard the evidence. I'm also happy that Jaime is not going down as a murderer.
I still wonder why she was out there that morning.