6 gauge is the best to use and should be
required on all boats with trolling motors. I have no idea why any boat builder would use anything less, except for the fact that they save a couple dollars on each boat. And when you multiply that by tens of thousands of boats, they save hundreds of thousands of $$.
Seriously, if they'd use the 6 gauge wire and add $5 to the price of the boat, nobody would complain.