So why not let the judge rule in the first place? My mum let my sister pick the colour for the new paint in our house when she was 16, and then repainted it afterwards because lime green was nauseating. Why let her pick to begin with then?
1) Defendants tend to be poor, and the juror's box is usually dominated by the poor/lower middle class. By contrast, there are very few poor people become judges, and I'd be surprised if there were a lot of poor people on the panels you described that selected judges. It's one way the poor can make themselves heard in the judicial system.
2) It allows for jury nullification.
I agree there are also downsides. Let's not pretend that the the issue is one-sided.