Google is still a company where (especially for projects like this) the work is done by engineers volunteering time outside of their core job description.
It may be as simple as "Morale dropped too low for the folks there to feel like it was worth it to continue," or "They let go of the people who were volunteering their 20% time to make this happen."
And spending time even if it's ostensibly "your" time outside of your core job on something that no one up your management chain cares about, or worse actively disapproves of you working on, isn't the best place to be in.
It may be as simple as "Morale dropped too low for the folks there to feel like it was worth it to continue," or "They let go of the people who were volunteering their 20% time to make this happen."