In my experience (I'm an instructor at one of these schools), the vast majority of incoming students already have a 4-year degree in a different area. I think for many people who may already be at a point in their life where they are paying off student loan debt, starting a family, etc., going without a significant income for another 2- or 4-years would be completely out of the question whereas a 3-month program is something they may be able to weather.