As someone who worked in MSM for 20-plus years (albeit in Canada, where entrenched political polarization arrived five years later than in the U.S., as with most things exported from the U.S.), I will suggest it was less about political pressure and more about not wanting to give credence to conspiracy researchers, lest the editors and publishers topple off of their journalistic ivory towers.
Any suggestion that the government's official line on the JFKA was off would imply endorsement of conspiracy, and most publishers I knew would rather stop drinking than put that in ink.