The best explanation for why Americans in the north and east were less supportive of the War of 1812 is:
They relied more on British trade.
The northern and eastern regions of the United States at that time had stronger economic ties to Britain and were concerned about the negative impacts that war would have on trade. This economic interest made them more hesitant to support the conflict compared to other regions that were more willing to go to war for issues like national honor and expansion.