Because it is between Halo 2 and 3 really.
"ODST takes place in the 26th century, where humans under the command of the United Nations Space Command (UNSC) are locked in a war with a theocratic alliance of alien races known as the Covenant. During the events of the 2004 video game Halo 2, the Covenant discover the location of Earth and launch an assault on the city of New Mombasa in Africa. Though the UNSC manages to repel most of the fleet, a single ship lands above the city and eventually retreats via a slipspace jump, creating a shockwave that destroys much of the city. While the rest of Halo 2's storyline follows the carrier to a ringworld called Halo, ODST focuses on the aftermath of the shockwave, where the Covenant still occupy the city."