Had law enforcement originally asked for it (and given them a subpoena), I'm sure they would have cooperated right away. I'm assuming law enforcement didn't ask them, and I would imagine that the reason behind it was they were able to ping her phone. Like I said, I don't think that any data they do get will be anything new, given that the police already pinged her phone. The family also got access to her phone early on through whatever hacking thing they did, so they didn't need the GPS data. The reason that it is being brought up now is because they are desperately trying to limit the search area.
As
@Suglo and
@kgreg2529 pointed out, Google has no way of knowing who is missing voluntarily or not, and they can't give out the data without a subpoena. I think it's the opposite of what you're saying. If Google "owned" the data, they could do whatever they want with it. Instead, Google allows **you** to keep control of your data, and part of this is that they will not release your data to anyone. Even if Samantha were found using that data, she could turn around and sue them for breach of privacy, and she would win a lot of money. These are the terms that everyone agrees to when they sign up for Google (or Apple), and why they invest so heavily in encryption. Any data that is used is either used internally or scrubbed of all personal characteristics before being sent on. I believe the only case in which they would make an exception is if the missing person is a minor. Companies prefer not to take that risk, and they wait for a subpoena. That being said, subpoenas are actually fairly easy to get, even for a layperson. But given that the police are still investigating this as a disappearance, it might have to come from them. Either way, I don't imagine that Google will budge on the subpoena issue.
While in this case it might make sense, I can imagine other circumstances in which simply issuing data because someone is missing would not be a good idea. What if it were a situation in which a domestic violence victim fled their abuser, and their abuser wanted the data? This is why it is better to have it go through the police department's hands.