Why do I need to isolate the ground and neutral in a subpanel when it is already bonded back at the main panel?

Take a look at the stickly grounding Bonding and surge supression in the Electrical & Lighting link at the top left.

If you still don't understand, holler.

In essence, the MUST be only one point of reference. That point, must be a signal reference, like water pipes, bath fixtures, CATV etc and it also must be the common point where power originates for the home.

Differences in potentials cause by ground currents (ground loops) raise the voltage on the points that are supposed to be zero.

Ground is both protective and signal ground. It must be potential free except during faults. A fault would raise the potential of the wire carrying the fault. If all of them terminate in one place, then the entire system's ground potential doesn't raise.