Great question. It is easier to rely on existing transmission at Intermountain than it is to build a gas turbine in LA (along with whatever infra is needed to provide a reliable supply of fossil gas to the generator site). You can even add batteries, solar, wind etc in the future at that site; coal sites are being remediated and turned into battery storage colo in many situations to rely on that existing transmission infra.
Related:
https://en.wikipedia.org/wiki/Path_27 (first two paragraphs are relevant to total transmission capacity to LA from Intermountain, ~2.4GW at ±500kV)
https://en.wikipedia.org/wiki/Los_Angeles_Department_of_Wate...
https://www.eia.gov/todayinenergy/detail.php?id=64604
(consider the fungibility of electricity vs other energy mediums)