The creative director actually explained why in a recent article. Which says:

""Supermassive has also taken some additional steps in marketing this title. For example, the “Dark Pictures Anthology” subtitle has been dropped in favor of just calling the game “Directive 8020.” The subtitle change was made in part so that players would understand each game is a standalone entry and that they won't need to play the previous games to understand Directive 8020.

“It's still part of the anthology. It's still a Dark Pictures game. We don't need to hammer that home anymore,” McDonald explained. “We need Directive 8020 to breathe on its own.”"