Justification is provided by the HSRA authority on their website here.
But many still question this decision. Among those are the California Rail Foundation (CRF), the Planning and Conservation League (PCL) and TRANSDEF that have challenged this decision in a lawsuit. This article written for the Sacramento Bee in 2004 highlights much of the background and arguments associated with this hot topic as they were known then. More recently, CRF, PCL and TRANSDEF hired French firm Setec Ferroviaire to study the routes to see which one made the most sense. In a news release, the report titled "Evaluation of an Alignment for the California High-Speed Rail Project Bay Area to Central Valley Segment" concludes that the Altamont Pass should still be the preferred route - the basis for the lawsuit.
Worthwhile reading on the topic can be found in Richard Trainors "Paradise Lost" found in the HSR History section of this site.
Much of this decision hinges on the ridership numbers, which were peer reviewed by UC Berkeley in June 2010 at the request of the legislature. UC Berkeley's findings draw into question the validity of the HSRA's choice to switch from the Altamont to the Pacheco Pass.
Kathy Hamilton wrote a multi-part article for the Examiner following the findings of the UC Berkeley review above. The third part in that series, an interview with Elizabeth Alexis of CARRD who first pointed out the issues, and Professor Samer Madanat from UC Berkeley, deals with the Altamont versus Pacheco Pass issue. See the link to Kathy's other HSR articles in the side bar under "Media".