Because it really doesn’t work. By all means you can have a local passenger service on it, but you can’t extend those stopping trains any further west than Wigan, or the journey time becomes too long and they become too overcrowded.
We probably also want to facilitate growth of rail freight from the Port of Liverpool, especially in opposition to Sefton Council’s untenable plans for it’s Rimrose Valley link road, which it thinks should built through what is currently a country park to instead grow road traffic.
You don’t need to use the Atherton line for Liverpool - Manchester expresses, but it’s not good enough as it is and would be worse as a tramway.
But it hasn’t found a new role. Of the several roles it had, Liverpool/Southport Express, Bolton avoiding, coal, Port of Liverpool and suburban traffic, it has been stripped of all but the least lucrative and now fulfils that role badly, while the others are either fulfilled badly by other lines, or road transport.
I’m under the impression that if a Metrolink service on the Atherton line was to start, that those stations closest to Manchester, or similar ones would inevitably reopen, which would improve the suburban service a lot, but degrade the line’s use for all other purposes.
It’s one thing saying the “golden age” of railways has passed, it’s another trying to justify the existence of infrastructure purely for a half hourly all stops Sprinter and then repeatedly proposing to downgrade the service yet again, just because it’s convenient.