Since PHP picked up the slack, diverging from it makes complete sense. When HHVM was released, PHP's implementation had a ton of issues, but now it's doing a lot better. However, even with PHP 7, the language itself is still riddled with confusing behaviors that will likely remain for a long time. They may as well break away from the cost of maintaining backwards compatibility and leave that to PHP, while they fix deep issues with the language and add features like typing.

I still don't see myself using it, but it seems like a reasonable path forward for someone who wants to modernize a large PHP codebase and would like features like type safety.