Perhaps they did not know where the Terran homeworld was when they were in Ross 128, or perhaps Vasuda Prime was a higher priority target for them.
* GTD Bastion
* Sirius System
The Shivans have finally determined the location of the Sol system. The Lucifer has moved into position here in Sirius for the subspace jump to Delta Serpentis.
From Delta Serpentis, she will certainly make the jump to Sol to destroy our homeworld.
How the GTA knows this is unknown, but that's the best canon explaination we got...
As far as non-canon goes, the Shivans place great value on the control of jump nodes (as evidenced by the Command Brief-cutscene) as such they probably went after the most highly travelled nodes first. Considering that the T-V war at the beginning of FS1 is taking place closer to the Vasuda-system than Sol, it is highly likely that most node-travel would have happened there (due to the location of the front lines).
Related to that it is also possible that the Shivans went after ships frist, which too were concentrated at the front lines, which again were closer to Vasuda Prime than to Earth.
(Of course that assumtion is based on a supply chain for the GTA that distributes from local centers (which in turn could be supplies by another more central location like Sol), which would have more intensive ship travel at the front lines - e.g. only one convoy is required to get supplies from Sol to Antares, but the distribution in the Antares-system would require multiple convoys from the local supply center to all units in the field)