In 2008 the NEC will require all sub-panels to have separate neutral bars and ground bars. There is one exception that exists at present that is allowing a 3 wire (H-H-N) feed to a detached building.
So you asked the why?
It's called objectionable current. We bond the neutral and ground at the service equipment because the only wanted
return for fault current and system current to the source (transformer) is the service neutral. This simply means virtually all current that returns to the main disconnect enclosure will use the service neutral due to its low impedance to the source. It will not follow the grounding wires in any appreciable value
to the dwellings grounding electodes which are also connected to the system neutral bar. This route has too high of an impedance so kirchoffs current law rules here.... the current will almost entirely follow the service neutral back to the source. However don't confuse what I'm saying to imply current will always take the lowest path of resistance. It will take all paths but most will take the path of lowest resistance. If we open the service neutral somewhere between the transformer and the main disconnect then the only
path back to the source is along the grounding electode conductors bonded at the neutral bar of the service equipment and connected to the water pipe and grounding rods or whatever your using for an electrode. Then current will use this path of higher impedance because it becomes the only path. It will then flow through the earth to the ground rods of the serving transformers. This is absolutely not wanted as you can guess. See below graphic for illustration. Take note that you lose your ground fault protection in a service neutral "open" condition and a breaker will not trip as shown. This is because the impedance is so high on the return through earth that current flow is too restricted and there isn't enough current coming through the breaker on a ground fault to trip it.
When we go further downstream (load side) of the main disconnect as to a sub-panel the same occurs if we maintain a 3 wire feed or a 4 wire feed bonded ground and neutral or not in the sub-panel. This is because all current on the load side... objectionable or not... ends up back at the main disconnect where it will "choose" the service neutral to get back to the source. So if we run a 3 wire feed to a detached building sub-panel we want the neutral and ground bonded because the only path back to the main disconnect panel for current whether it is fault current or system current is the feeder neutral between the sub and the main disconnect. There is no equipment ground with the feeder so only one path exists. Note:
All sub-panels that are located in the same structure with the main disconnect must have 4 wire feeders.
Ok so now lets look at a 4 wire feed to a sub-panel. Only difference is we run an equipment ground wire or metal conduit along with the neutral and hot wires (H-H-N-G). If we bond the neutral and grounds at the sub-panel neutral bar we have now created two paths back to the service equipment. Both paths are low impedance (copper wire or aluminum wire or metal conduit) So return current from the branch circuits served by the sub will pretty much split evenly.....half on the equipment ground and half on the neutral as it returns to the source. So if we would have an open neutral of the feeder to the sub all current
will now follow the equipment ground because we bonded it to the neutral return
at the sub-panel. Again Kirchoffs current rule wins. We do not want any current on the equipment ground except fault currents and these will exist very briefly because they will be stopped when the circuit breaker opens. Equipment ground is for human safety so we dang sure don't want current hiding on the equipment ground unknown to us. Especially if we are using metal conduit as our equipment ground. So we do not bond safety ground to the neutral bar in the sub-panel when we use 4-wires utilizing a wire for equipment ground or 3-wires with metal conduit as our equipment ground. As the below graphics show.
4 -WIRE or 3 WIRE with metal conduit as equipment grd
to detached building only method allowed as of 2008 NEC