Others have pointed out the concerns around negative reviews of things still subject to change, but the other aspect is just the relations with media.

I'm sure tons of journalists have been playing. And probably even working on content covering the game, but not publishing it yet. Once valve is ready for coverage they'll have polished content ready. And valve can control the timing so that coverage happens right when they want the hype like maybe a few days before an open beta.

By covering it early you encourage other journalists to do the same, rushing out low quality content to get the views before others do. And for valve to not let any journalists see the game early to avoid this.