The longer the project exists and the more people there are who get to try it, the more likely it is to leak.

To speculate, here are some reasons why keeping it secret longer might be hard:

  - They're going to do wider testing within Google.
  - They're going to start bringing outside testers in to try it.
  - They're going to start working with manufacturers.
  - Some newspaper got wind of it and is about to publish a story. (I think this happened with driverless cars?)

Apple is better at keeping secrets, and even they leak.

Also, it's nice for the people working on it when they no longer have to keep what they do a secret.

Edit: although, in this case, the timing mostly has to do with Google I/O starting today.