Easy.

High Charity suffered severe structural damage while escaping from the battle around Installation 05. The Flood was enacting repairs and overlaying fleshy growths over the superstructure, but had insufficient time and resources to complete the repairs. High Charity would have broken apart attempting to enter the Voi Portal. If tidal forces did not split her asunder, the various stresses from atmospheric entry would do the job. Even the process of exiting slipspace would have pushed the dead city to its limits. Leaving a rear guard in the form of an infected Earth was a worthy objective, but attaining it would strand the Gravemind short of the Ark.

But the Gravemind is clever, and possesses the collective technical knowledge of an extinct galactic civilization. The back-up slipspace drives on High Charity were modified and upgraded during the journey to Earth, and Gravemind entered the Voi Poortal directly from Slipspace.

A halfhearted attempt was made to establish itself on Earth, but most of the Flood-captured warships were sacrificed around Installation 05 in the defense of High Charity. One was sent ahead to Earth as a scout, and what few remained were needed to escort High Charity to the Ark. The invasion took the form of dropships and civilian vehicles that were shunted out of Slipspace in Earth's orbit, to be quickly intercepted by Earth's remaining defenses and those Separatist ships too damaged to accompany Rtas 'Vadum's fleet.