That's how the system works

And it does not come by randomly. If you are really interested in knowing the history and rationale behind it, maybe you should do some research on your own or talk to someone. In short, the commom wisdom is that the lower court judge who actually presides the trial, questioning the P/D, seeing the witnesses, etc. stand a better chance in fact finding than any appellate judges who merely review paper. Also have you heard of "collateral estoppel"