Recent comments raise a new question:
Why were the East-Coast North American Indians, sitting on pretty good land less advanced than the Mexicans?
North American natives had very little in the way of useful indigenous crops that could be domesticated. Most of the crops associated with the Eastern Agricultural Complex were abandoned after Mesoamerican crops spread that far north - Today many of them are considered weeds (like sumpweed, pigweed). They're characterized as having small yields that are very labor-intensive to harvest and some of them are even major allergens. The only ones that are still cultivated to any significant degree today are sunflowers, squashes, and gourds (though the latter two were also domesticated elsewhere as well, leaving sunflowers as the only major contribution).
Wild rice has also been long been cultivated around the Great Lakes region, but it has a pretty limited range from what I understand. There's also a variety of indigenous plants that are widely-cultivated today (maple, cranberries, blueberries, Concord grapes, Jerusalem artichokes, pecans, black walnuts, mesquite pods, etc) but they're not enough to power large populations of people by themselves.
Most of the productive food crops in the New World that could support large populations originated south of the United States and had to spread north over many different climate regions (desert, plains, mountains, woodlands, etc), which slowed their dispersion and crippled the development of North American native civilization.