The answer to this can be found in Chapter 8, page 152 in the paperback edition. There, Diamond explains that the climate of the Eastern US was not enough to make up for the lack of really good resources there. Your question states that there were natural resources, but those resources were not really enough to make food production as easy as in places like the Fertile Crescent.
Diamond specifies a number of problems with the Eastern US:
- Its wild creeals were not as useful (smaller seeds, harder to cultivate) than wheat or barley.
- It had no pulses.
- It had no fiber crops.
- It had no fruit or nut trees that were usable.
- The only domesticable animals were dogs.
Because of thse shortcomings, the Native Americans were unable to develop food production as early as people in other places did.