Because 'school choice' is a loaded political term for defunding the public school system these people depend on so that the money earmarked for these systems gets handed out to private or charter schools, thus leaving the public system in a worse place, especially when the public system must take the worst students while charters can dismiss them. In other words you're socializing your losses and privatizing your winnings.
Even then, the "opposition" is more like healthy skepticism. Even in liberal Chicago we have a charter school boom and thus far the results aren't impressive especially when you consider they benefit from the 'cream of the crop' issue defined above. They can dump the low performing students onto CPS while enjoying the test scores and metrics more motivated students or the students without learning disabilities or the students when more involved parents can produce.
But no, we're not going to defund CPS entirely overnight and hand out its budget in its entirety to charters. That's not "opposition" that's sane thinking. And yes, if charters can't deliver the goods and if they cannot deal with the worst students or improve them, then they have failed and shouldn't be subsidized by the taxpayer. My tax dollars shouldn't be an entitlement to those with experimental and untested educational theories who can fill out an application to create a school. You have to earn it by showing your methods produce results - with all the kids, not the cherry picked one. That's what they currently don't seem to be able to do. Even then we still give them money hoping they do someday or that the added competition creates a stronger educational dynamic.
That said, many charter schools are just a ploy to get taxpayers to fund weirdo religious schools. A lot of charters are extensions of churches/mosques/temples or ethnic groups (often with religiosity as the main motivator). That isn't exactly the education revolution you're looking for here in many cases.