ELI5: Why are ocean-going barges loaded fore & aft, but not in the middle

I live in Waikiki with a bit of an ocean view. There are regularly scheduled barges going to & from neighboring islands.

The barges will have big stacks of containers fore & aft, but none in the center of the barge. With a tractor trailer flat bed, I can see putting the weight over the axles, but what's the deal with how barges are loaded?