In raw economics terms, the price LA pays is lower than the cost of them opening their own coal plants, otherwise they would do just that. Therefore, it is for some reason cheaper to open a coal plant near portland (maybe closer to the coal source) and run a wire down to LA, than it is to come up with some other source of peak demand electricity for those 3 million people in LA who rely on this capacity. A private market does what is profitable, ultimately.
Maybe if we had public utilities, however, we would actually invest in 'unprofitable' nuclear energy and save our planet in the process, since we wouldn't be beholden to making shareholders a profit.