Ignoring the lines with only a few trains a day (Far North, Heart Of Wales, Heysham, Knottingley - Goole, lines that only see diversionary services etc), I'd suggest that somewhere with a messy service pattern may struggle to justify wiring.
For example, if you assume that anywhere with a self contained half hourly service should have a good case for electrification, there are still some routes that see a fairly frequent core service that has some irregular/ infrequent "branches" to it.
An example of that is the Calder Valley line. Plenty of DMUs each hour (four an hour from Leeds to Halifax via Bradford, four an hour on the section from Rochdale to Manchester) but the muddle of routes means that if you don't wire all of the lines then you don't get the full benefit of electrification. i.e. wire the main line from Leeds to Manchester via Todmorden and you free up some DMUs, but not the Mirfield - Brighouse - Todmorden line or the Halifax - Brighouse - Huddersfield line or the line from Todmorden to Burnley or the other side of the Tod chord...
...you'd need to either wire a few bits of track that don't have such a frequent service or cut links in order to justify electrification. So although it has a lot of trains, it may have a poorer business case to electrify than some less frequent routes.
Plus, places that see fairly extreme weather - would the Settle & Carlisle require specialist equipment to deal with the harsh Dent winters?
