Because while some Catholic parents would prefer to send their kids to Catholic schools, they would send them to public if the pricing was prohibitive. Orthodox Jewish schools, on the other hand, are aware that they can always count on the frum world never sending their kids to public school, so they can charge whatever they want and they do.
DY: I imagine you are used to areas where there are multiple Jewish schools and there is some pricing competition. In the out of town communities where there is just 1 MO day school in a hundred mile radius, the tuition is truly impossible for some (20K+ for kindergarten). If the parents can only find work in an area like that, they have a real problem. They are just “putting person comfort before their kids future.” It always astounds me how much of a “let them eat cake” mentality some–otherwise very empathetic–frum people have when it comes to this subject.