The reasons are many and varied, the most important probably being that God has ordained parents to be the primary teachers of their children.
Education is the parents' responsibility.
These fundamental beliefs keep a family homeschooling even when there are good schools in the area. Other reasons, such as academic excellence, continuity in communication of values, or financial constraints are real and important. I believe not. God designated parents to do the job of teaching. This assumes almost automatically that top- flight professionals are betters educators of children than parents, solely on the grounds of their level of familiarity with their subject matter.