It was not covered at the time because the way it was "released" meant that the claims were unsubstantiated and unsubstaniable. As more information comes out the appears to be actually verifiable (and verified), it is showing up in "completely mainstream media" outlets.
There's a problem when you try to release a news story using more or less precisely the same playbook that would be used if the story was fabricated. You can't blame the media for not taking you seriously when you (in this case Guiliani) does more or less precisely what someone who was lying through their teeth and knew it. If you want them to take you seriously, you need to give them the chance to verify your claims. The Hunter Biden Laptop story was bungled as much by its "releasors" as by the media.
Also, even today, it's not entirely clear what the big deal actually is, other than being able to say "possible corruption" and "Joe Biden" in the same sentence.