I have heard (granted, this is not confirmed in any way, shape, or form) that it has to do with lay people being more...neutral, or something along those lines. As well as that whole "beyond a reasonable doubt" thing. Though, honestly, I can't say I disagree with him. Lay people are far too easily convinced by skilled turns of phrase and whatever bullshit they see on TV crime dramas. Honestly, one should probably be given some kind of competency test before being allowed to serve on a jury to weed out people that think CSI represents actual forensic work.