This is because conservatives are always looking for a way to make coercion more efficient. They don't like vouchers when vouchers clearly extend government power into areas where the government does things conservatives don't like, such as government housing and government food subsidies. But they are all for efficient government when it does something they like, such as tax-funded education. They think government will be less intrusive. They are wrong. It will be more intrusive.
-Gary North
http://www.lewrockwell.com/2013/08/gary-north/why-did-milton-friedman-want-school-vouchers/