Why they should let these counties split: They have no representation in the state government, at all. Most of the US doesn't even know that anyone lives there. Lots of local concerns that really need to be taken care of, that don't get handled thanks to moneys being directed to a high-speed train to no-where in the south.
Why they wont get to split: They own all of the watershed for the Sacramento River, and all of the Watershed for the California Aquaduct that provides 30% of LA's water and 90%+ of the central valley's AG water.
Greedy bastards.