Why are almost all Wisconsin voucher schools religious?

Jack Craver:

Jim Bender, executive director of School Choice Wisconsin, the pro-voucher lobby, says the choice program is a reflection of the private school market, which in Wisconsin is predominantly religious.
“If you look at the history of education in Wisconsin, that’s a cornerstone of operating schools in the private market,” he says, pointing out that many parochial schools are typically cheaper than non-religious private schools because they are subsidized by their affiliated churches. “In the past, not long ago, many religious schools were free.”
Bender says he believes the development of a statewide voucher program will “change the economics” and allow for more secular private schools to flourish, since they can now receive taxpayer funds.
And yet, as I mentioned above, that hasn’t been the case in Milwaukee.
Alan Borsuk, who helped found two Jewish schools in Milwaukee and covered the city’s choice program as a reporter for the Milwaukee Journal Sentinel, says the money provided in the voucher program still isn’t enough to convince many secular schools to participate.