I suspect because in terms of the number of DMU cars that can be replaced with EMU cars per mile of track electrified it doesn't score that highly.
Electrifying one of the two lines* between Manchester and Liverpool was enough to allow the fast Liverpool to Manchester airport services to be electrified and to feed into a future transpennine electrification. Afaict that has left the CLC with just the stoppers and the Nottingham/Norwich train. The former only have a relatively small amount of under the wires running and were histroically relatively short trains, Yhe latter has a lot of under the wires running at the southern end of the route, but it would take a lot more than just wiring the CLC to let it run electric.
* And if you are only going to electrify one, the chat moss makes far more sense than the CLC, because the chat moss has a junction with the WCML and the CLC does not.