I think it might be they’re trying to protect their access to the Trump campaign. If the so-called MSM start writing the unvarnished truth, they’ll not only never get another Trump official on the record anymore, they’ll also probably get a lot fewer off-the-record leaks as well. Then they’ll be forced to write nothing but speculation pieces, and no media outlet wants to look like an outsider looking in.
On the other hand, they could write anything they wanted about Biden, and they would never lose their access, because if the campaign looks like they’re shutting out the public via the media due to a grievance, it will be a bad look to D voters; whereas the Trump campaign wields grievance-based withdrawals of access by the MSM as a badge of honor, and among their fans, they’re right.
It’s simply a different media landscape for each campaign.