Can you provide some insights why the connections between Google's data centers was NOT encrypted until now?

Unfortunately, I'm not sure I'm the right person to share more insight. I don't work on the network team but data between data centers flow on our own network. Data between a client's machine (machines on external networks) and machines on our networks has been encrypted for a while. Data at rest on servers has been encrypted.

Before these revelations, the tech community in general didn't expect that we needed to encrypt all traffic flowing on our home/office LANs. Like the rest of the world, these spying revelations have taught us that we need to be much more paranoid than we were earlier and are now encrypting data on our own networks.

As a user of a lot of web services that are deployed on the cloud, I'd actually beseech my fellow tech community to do this too. All and any user data passed between any two servers (even on a backend, internal, local network) needs to encrypted.

Thank you for providing your insights, it is important to know that the data on the disks is encrypted. I know about the encryption (https) between the browsers and Google's services - Google was one of the first actually to switch the services to https.

But I have to say that I am still quite surprised that there is no encryption between data centres. Working from time to time for industrial customers, on business critical software, most of the time it is required to encrypt data between servers, even when the hardware is in the same building, because they are afraid of leaks/attacks from inside.

I think Google has to do some explanation to the public about their security. Though I do not know if it is not too late for some google users.