Journalists are trying to find places to move over their audience since twitter put their audience behind a login screen

Same with news outlets, government accounts, agency and municipality accounts etc. Forcing login was such a tonedeaf move. Twitter was a broadcast platform, the internet soapbox, now it can't be and there's no reason for those accounts to stick around. Previously those entities could assume Twitter was essentially the internet's town centre, now they can't.

I wonder if it was an attempt to cater to advertisers concerns over bot traffic inflating ad numbers.

Journalists are not that critical to it. They actually get paid for contributing to other sites, some of which Musk has now banned like Substack, and they specifically don't get paid to tweet.

There are other large communities on Twitter. These include sports, semipro artists, the nation of Japan, people in politics, and a variety of assorted insane people and scammers like crypto, VCs and vaccine skeptics.

The reason it's failing is that the last group now owns the site.