I doubt there's any one answer, but what my various hypotheses share in common is the suspicion that those covering the story let questions about how the story would be understood get in the way of finding out what the story was. Rather than be driven by the desire to know the truth, and to speak that truth to power (whether that power was in Washington or Wuhan), I fear a brake was applied to serve some ill-defined social interest, even if the only interest was backing the public health authorities that we were all relying on to get us through the pandemic, and who had been quick to affirm the story that the virus was of natural origin. But that's public relations, not journalism, and doing public relations is not the way you build trust, either in journalism or in public authorities. Good journalism is driven by fundamentally anti-social forces: skepticism of conventional wisdom, distrust of authority, the determination to ask unpleasant questions and to refuse to accept comforting answers. Those anti-social forces have pro-social effects only if applied universally — not because all authorities are equally deserving of skepticism but because all deserve it to some degree, and playing favorites doesn't improve public confidence in the favored authority but only diminishes public confidence in professional journalism. You can see the consequences of the opposite approach in the mainstream media stories that do cover the lab-leak hypothesis. Because the subject matter has been assimilated into America's culture war Borg, authors need to step gingerly around their audience's perceptions that only haters and cranks would believe such a thing. That's a perception that the mainstream media helped create; it's no surprise they are reluctant to challenge it directly. But it's their job to do so.