Vague "educational reasons" is always the noble-sounding excuse they use, but often if you dig deeper they'll admit it's more about the various forms of separatism.

Sometimes you don't have to dig. A ton of moms in my wife's church group permanently pulled their kids out of public school in recent years, and they will openly admit that it's about keeping their kids away from "those" people, where the definition of "those" runs the gamut.