Subterranean termites living in the soil will have instant access to the wood in your home.

Also the soil can get damp from rain/poor drainage and cause mold and mildew problems.

A slab adds a physical barrier to prevent this but it is very difficult to modify or repair anything built into the slab. For example the drain pipe.

The crawl space acts as an air barrier and allows for easier access to plumbing or HVAC systems. It also takes less concrete since you only pour in the perimeter of the house for the foundation.