It’s not that the media wanted to avoid helping Trump or hurting Biden, even going so far as to withhold reporting the truth of the pandemic’s origin. But rather that the media had a higher threshold of evidence for publishing accusations of malfeasance against another nationstate than Trump did, and hadn’t met that threshold till recently.
Trump was notoriously flippant, with low or no standard for truth, or even merely diplomatic tact. His MO is not truth-finding but rather saying anything distressing to his political adversaries. He is an archetypal Internet troll with the soapbox of the US presidency.
He spent four years doing that, essentially crying wolf over and over. By the time the pandemic hit, the media was universally resistant to serving as a megaphone to amplify his trolling.
Thus they held back on this story until they could piece together and verify enough of it to meet their higher threshold of evidence. That simply took until now to do.