Lack of annual slash-and-burn in North America?
Slash and burn has been used in North America and still is at times.  I see regular field burning near me where fields are particularly rocky and tilling if impractical for the crops grown.  It also occurs when there is intrusions of some noxious weeds that are controllable through fire as effectively and much cheaper than chemical controls.

In general, I would consider slash and burn in disfavor because of it consumptive nature.  The practice does provide some otherwise missing nutrients, but it consumes large quantities of biomass while doing so.  More favored in many regions it to use that biomass to supplement the soil by tilling it in and allowing it to aerate the soil while slowly breaking down and augmenting the soil.  Between being fortunate enough to have good soil to begin with, or the ability to augment with either natural or chemical supplements, many North American farms do not need the quick fix burning can supply so opt for the slower, longer term solution of turning in the organic materials.