Because starting something like this is exciting, and there is no reason to keep it secret. I can't think of a good reason to not announce. It's not like they called a huge press conference or brought this out at a trade show, it's a quick blog post. Do you not tell your friends when you start some cool new project? People like talking about this stuff.

Also, more practically, ideas for this kind of venture can come from anywhere. Maybe someone reading this has a brilliant idea for improving city life, but doesn't have the money or entrepreneurial ability to make it happen. Maybe some startup has a great product in this area, and could really benefit from a partnership with a company with Google's resources. Maybe some city government wants to commission a project to track and optimize their energy usage, I don't know. Why would you intentionally work in isolation when there are potentially great ideas out there?