It's very simple - there is no money (unless the DfT explicitly approves it).
Work is proceeding on MML (Kettering to Market Harborough), North TP (Colton Jn-Church Fenton) and in Scotland (East Kilbride/Barrhead).
The TP scheme has some legs and is very likely to extend westwards (but will not include the Calder Valley or Hope Valley routes).
TfW is electrifying some of the Valley lines round Cardiff (some of it with discontinuous electrification).
HS2 will be electrified from Day 1 (contract in the pipeline).
NR has published its decarbonisation strategy, including a detailed list of which routes it is considering wiring, but the DfT has not approved any of it yet.
Traction Decarbonisation Network Strategy - Interim Programme Business Case (networkrail.co.uk)
Alternative modes of traction are the rage - bi-mode, battery and hydrogen projects are in progress, mostly at the experimental stage.
Crucially, NR has yet to prove it can deliver electrification at a price and timescale acceptable to the DfT/Treasury.
Most routes also need a major upgrade before wiring is worth doing, which weakens the business case.
The Chancellor might give some clues about rail upgrade policy in the Budget tomorrow - but again, he hasn't any money to splash around on a railway running at 30% usage (if that).