Why is uranium enrichment necessary for nuclear power?

The Canadians developed a reactor, I think it was called the CAN-DU (Cute, hut?), that used heavy water (Deuterium oxide, hence the "DU") as moderator/coolant that could use native uranium ore as fuel. Of course using that much heavy water is obscenely expensive. Enriched fuel increases the likelihood of a neutron hitting a U-235 rather than a U-238 atom thus making critical m/geometry possible on a practical scale with affordable moderators and coolants.