Because it basically leaves children from poor/lower-middle class backgrounds up shit creek with little hope to get out of the cycle of poverty by educating themselves. As things stand the public education system is so gutted and twisted seven ways to Sunday that many classrooms only function because the teachers pay out of pocket for supplies the schools can't afford (because many school districts have fallen prey to the Libertarian modus operandi of ensuring a public system is too broken to work so they can point to it to justify a Randian drive to privatize everything on the taxpayer's dollar).
This is only going to get worse under DeVos, whom has never attended or sent her children to public school and is thus as uniquely unqualified to serve as Secretary of Education as Trump is for President or Carson is for Urban Development (or Bannon is for anything with a security clearance higher than gmail.com), because she wants to defund public schools and use taxpayer money for religious charter schools and exorbitantly-expensive private schools.
Even assuming financial aid in paying for tuition, the poor/lower-middle-class are going to be left in the breeze (just like every other time hard-right conservatives take over despite consistently being able to convince those same demographics privatization is in their favor) because there will be many families that, even with vouchers to help pay for school, won't be able to afford the remainder of tuition, to take time off work to get their kids to/from (which is currently handled by a public bus system, which would also be privatized under DeVos's plans to defund public education), and pay for uniforms and any other supplies the school doesn't provide (because private schools are primarily for-profit organizations like any other business that gets in on government-run markets).
The sad thing is, so many people buy into that trap--where an existing public system is run so terribly by people with a vested interest in privatization that they can point to it to justify privatization--that this shit happens over, and over, and over, like clockwork, and because Americans have a unique misperception that their poor aren't an exploited, impoverished working class but millionaires just one lucky break away from their fortunes, they never wise up to it.