Private schools of any type can be expensive although christian schools do tend to be less expensive than other private schools. For many families the $200 to $500 a month tuition (per child) is beyond their budget. In regards to "making Christian schools free", land, buildings, teachers, and support staff all cost money.
When looking at both standardized college entrance exam scores, and national education related competitions (science, writing, spelling bees) invariably a disproportionate number of home school kids are taking top awards because they are getting a better education than they would even in private school.
I've had the discussion with numerous home school naysayers over the years but the numbers don't lie. When a home school teacher (Mom) has between two and six students (versus an average of 15 to 20 students per class in private schools and 30 plus students per class in public schools) she can give each student more one on one time and custom tailor class time to each child's needs in a manner that neither private or public schools can. Likewise, in home schooling, education tends to extend well beyond the "walls" of the classroom.