I think it’s a little like “no one goes there anymore it’s too crowded”. If people started posting on mastodon people would use it, and it wouldn’t be so nerd-dense.
A friend and I had a minor fight about this. She was like “but all the good content is on Twitter” and I was like sometimes you have to be the change you want to be and suffer a little to make the world better. I think you can suffer through less immediate memes. She did not accept this.
But anyway yeah you’re right that content needs to move. NPR and others could probably just switch, but none of them probably want to be the first to move.