It's about trust and credibility, of course. When global warming scientists tell us every 10 years we've got 8 years to save the planet, the public stops listening. Yes, I think climate change is a thing. No, I don't think it's a near-term existential threat. Or when the APA questions the mental health of gun owners but says in the same breath that cutting off your genitalia is a health decision we should respect.

Moreover, when I see the Twitter feed of Dr. Climate and it's filled with rantings about the glories of socialist Venezuela or some other absurdity, it's hard for me to go "oh yeah, I'm sure this war waged against American business is entirely about saving the earth and not at all colored by your loathing of American capitalism." Is that fair? Not necessarily, but am I really supposed to go "oh well he's got a PHD so I should ignore that he's a fringe radical because he can separate that from his real job"?

Access to information plays a role, of course. People distrust the government more because they have more access to it, and see how it's largely filled with self-interested sociopaths. And social media, as you indicate.

On that note, I also think a 24/7 news cycle that is now 90% polarizing opinion-driven has rotted our collective American brain. Millions of Americans just sit and watch Fox or CNN regurgitate the same "you should be so mad about this!" shite all day every day. It's a drug, and people crave it, and to deliver it Fox and CNN have to turn "experts" into "experts who take absurd positions for views". So subject matter experts no longer have an interest in remaining neutral on overt political matters (and some of that falls back on us - the viewing public).