It's to do with politics though because politicians in UK govt, who control those lines, refuse to pay for the cost of electrification of those lines. Note that in 2012 the plan for the Valley lines was to electrify the entire SE Wales local network, including Ebbw Vale, Maesteg, Barry Island, Penarth, and the VoG line. Then a row erupted over who should shoulder the most cost between UK Govt and Welsh Govt, and by 2014/15, it was clear that there was only enough money available to electrify the 'Core Valley lines' on a constrained budget.
If UK govt had really bought into the South Wales Metro scheme, they would offer to at least electrify the lines to Barry, Penarth and the VoG so that tri-mode electric/diesel trains are not needed for the Rhymney line, and the trains could operate on electric throughout. As it is we've got the crazy situation where platforms 6, 7 and 8 at Central won't be electrified because NR don't have the money to do it, whilst the rest of Cardiff Central is electrified.
So the entire electrification of the Cardiff local lines and the Valley lines has all been about politics since the start 8 years ago. Because the Labour Welsh Govt in Cardiff and the Tory London govt continually row about funding and devolved responsibilities, we have the compromise scheme we've got now, rather than the full electrification of the entire local network that was proposed 8 years ago.