Because public is only OK if it's done by Republicans. I know this sounds flippant and partisan, but what's the serious counterpoint? Spend publically on war, on tax cuts (1), on anything favored by conservatives: it's a natural function of government. Spend on anything else: it's usurping taxpayers rights to make their own choices, and reducing efficiency because everybody knows that individual Americans spend their money more wisely than the government.

(1) yes, i know that the riposte: "tax cuts are not expenditure, they are giving money back to tax payers, where it originally came from".