The other explanations here don't explain the long delay between the start of the investigation and the release of the footage. Yes, storing customer data is what we'd expect from Google and yes, the FBI can coerce Google to provide this data for their investigations. But it does not take a week for Google to find a file on their servers.

My hunch is that Google initially tried to play dumb to avoid compliance, as to not reveal they do in fact retain customer data. They had a plausible excuse as well -- the owner had no subscription so they don't store the data -- and took a gamble that this explanation would suffice until the situation resolved itself. I suspect that authorities initially took Google's excuse at face value, since they parroted this explanation to the public as well. As pressure mounted on authorities to make some headway on the case, they likely formally exercised whatever legal mechanisms they have at their disposal to force Google's hand, and only then was the footage released.