Slab construction versus building a foundation to create a crawl space is a area where builders can save money that can be passed on to the buyers. In todays economy where new housing has taken a hit, keeping costs down counts.
As far as from a build point of view, I find that people who suffer from any kind of foot, hip or knee ailments prefer crawlspace construction as they claim walking on a concrete based floor bothers them more. From any type of repair issue, being able to get to your water or septic pipes without breaking through a floor is easier with crawlspace construction plus crawlspace allows a home to breath better. Crawlspaces if large enough also add storage space for items such as lawn & garden tools, window screens during the winter months, bicycles and items not prone to being damaged by moisture.
The plus side of slab besides saving dollars at purchase is that in NC, we are known for termites. I don't see as many termite issues with slab as I do with crawl spaces mainly because all areas accesible to wood products of a home are more easily viewed. Very few people take the time to have their crawl spaces inspected on an annual basis.