Because this is nothing more than attempt to add 4 more Senators that have a chance of being won by Republicans. It's the same reason why "Southern California" includes OC.
This would add a net of four more Senators, which only might go Republican. Considering that California is generally considered solidly blue as it stands, this would almost certainly result in the balance shifting toward the Democrats in the Senate.
My point is that the way the state is divided in this proposal all but guarantees that all three will be blue.
Right now the electoral college votes in CA are basically guaranteed Democrat votes.
This would break that, affording at least 1/3rd of them to Republicans.
That's huge, and would likely ensure a Republican president for decades.