Because only House Republicans oppose her mandate.

Via The Hill:

…The first lady went on to attribute opposition to her controversial school lunch program to partisan politics in Washington. House Republicans want to loosen restrictions requiring schools to serve meals with less sodium, whole grains, and reduced sugar and fat, saying the requirements are straining school budgets — and that children aren’t interested in eating the healthier meals.

“That’s really saying something,” Obama said. “I mean, for most folks in this country, making sure that our kids get decent nutrition isn’t all that controversial, because as parents, there is nothing we wouldn’t do for our kids -— nothing.”