Mercantile Republics

The United States was a republic. Its very breath spread republicanism across the globe, and ushered in a new era for civilization. However, when technology relapsed, so did society, and America is now a continent of barbarian tribes and feudal overlords. Still, there few polities here and there that abide by rule of law, but still much more oligarchical than their 20th century counterparts.

Geography

Republics tends to spring from city states, usually found on peninsulas, or sandwiches between rocky mountains. Historically this has included Italy, North Africa, and even the Swiss Alps. In America, these would be the Northeast and Northwest. These allow a certain natural defense from outside conquerors, at least for a little while, and teach these relatively small populations the art of self-reliance. The narrow amounts of land limit the manorial system, and the lack of natural resources incentivize a more craft-making economy. As a result, merchants and guilds run the show, as opposed to  landowners.

Continuity

The Northeast's clinging to its Republican past isn't necessarily one out of idealism, but inertia. As a political institution, the United States was a republic, and it stays that way, if only because the members Senate and House of Representatives were not about to relinquish their power. New Englanders were also used to a more pluralistic government, with individual townships and their own town councils governing, as opposed to counties which, by their nature, more easily reverted feudalism. That New England communities were some of the earliest meant that municipal structures were more built to human scale, and so an ambitious warlord was less likely to come and take advantage of the chaos.

The Church

The concentrated presence of the Non-Denominational Church throughout the Northeast has also kept warlords from carving at their own slices. The Church was built upon republican ideals, and besides, they don't care for warriors coming and making messes in their backyards. The inordinate amount of Churchmen also means an inordinate amount of literacy, and the learned are much more likely to demand representation, limited as it may be.

Cascadia

The valley between the Cascade mountains is much more fertile than the Northwest, and could make for a very robust manorial system. But it is relatively narrow, and the residents of the once Oregon territory come from a strong tradition of hippie idealism and silicon valley-style entrepreneurialism.  This of course, has led to a warring clash of city states. But in general, they try to avoid feudal west of the mountains. The skillfully belligerent are sometimes granted land along the Columbia River, sort of to be "kicked upstairs" to a buffer state.