If I had to make a guess as to why it was done the way it was, I'd say that giving each island completely to one country would concentrate the fighting too much at the borders, which both reduces the options available to the players and increases lags and warps. Giving each country a piece of each island spreads out the fight to the point where it doesn't overwhelm the ability of the players' systems to handle the clutter.
Also, giving each country a whole island pretty much destroys the ground game until you capture enough of another island that they can't just pound the VHs on the captured fields to keep you from launching GVs. If we had additional ships, like LSTs or LCUs, so that it was possible to conduct an amphibious landing, that wouldn't be as much of a problem.
An LST should be doable -- a moderately-sized, slow ship with a couple of small acks; you beach it bow-in, open the bow doors, and for a period of time (15 minutes or until sunk), it creates a vehicle spawn point on the beach in front of it.
An LCU would be a little more complicated to do; it would probably be most conveniently handled by arranging that if you drive a GV to a specific spot at a port, a function key makes the FE load it onto an LCU and put you at the helm. Then you drive the LCU to a beach, drop the ramp, and then the LCU disappears and you're put back in your GV on the beach.
