Any parent can tell you that a hungry child isn't capable of doing any complex activity. Attempting to educate a kid who hasn't eaten in god knows how long, for whatever reason, isn't going to work.
Beyond that, universal school breakfast and lunch help people who can afford it by removing yet another thing for parents to worry about. And it eliminates the stigma of only the poor kids getting free lunches.
Republicans have been disdainful of public education since before I was born, so they're trying to make it fail by continuing to spend money trying to teach hungry kids.