Forgive me if the very premise of my question shines a light on my ignorance as I, admittedly, have the barest of grasps on the topic. For the sake of clarity, assume that my question refers to the Early Woodland period.

I am familiar with locations like Cahokia and cultures like Adena and Hopewell, but when reading of the larger cultural traditions of this time period I am struck by the fact that despite (what seems to me) their large geographic spread, the centers remain clustered on the major river valleys like the Ohio and Mississippi. I am aware that there were large trade networks during this time that would have brought goods inland from the coast to major cultural sites like Cahokia, but my understanding is that larger cultural traditions (Adena, Hopewell, etc) are not inclusive of peoples populating the coastal areas.

Why are coastal peoples not thought of as having been a part of these cultures? Is there a larger cultural tradition that coastal peoples can be said to have been a part of? Were there reasons why coastal peoples might not have formed into a cohesive cultural tradition (a lack of population density, for example)?