There are pros and cons to each foundation type.
Slab foundations are generally recommended for more wet climates and help reduce mold growth in the home and are less expensive for the most part. In addition, they tend to be more energy efficient as they don't have to be heated. On the flip side, they are more prone to cracking with shifting soil and can be damaged by tree roots.
Crawlspaces work well for several applications. Sloped lots and areas prone to earthquake and ground shifting, they are better for drier climates as they are prone to moisture and mold growth. Wiring and ductwork are more accessible and can be used as a storage area if finished.
They also have their own drawbacks. They are more expensive to construct than a slab, they take longer and are less energy efficient than a slab.