Later this week, world leaders will gather in New York at the United Nations to endorse international development goals for the next 15 years. It is the culmination of a four-year process for setting priorities to help the world’s most disadvantaged people—a process beset from the start by horse-trading, haggling and endless consultation. In a bid not to offend anyone, the new development agenda is expected to include an incredible 169 targets for investment. Giving priority to 169 things is the same as giving priority to nothing at all.

If...