I'd guess that it's a combination of things, but I think what sticks out to me is that a) they probably hit a plateau and felt like they needed to do something to break out of that (and didn't really have a good idea so they started "mixing things up"); and b) messaging seems like a product that everyone wants to solve, but doesn't necessarily offer amazing monetary returns.
I loved GChat. It was simple. I could use Pidgin or Adium or any number of third-party clients. It did what I needed. However, I'm guessing at some point, they were working on and maintaining a product that didn't really have the metrics that they were looking for. Growth probably slowed a lot once so many people were using it (so the honeymoon phase of "we'll figure out the money and strategy later" was over). Likewise, one of the great things about GChat was that it felt so un-monetized. There were no ads or anything. Compared to the alternatives of the day like AIM, Yahoo, MSN, etc. it just felt like this easy, clean, simple messenger that worked without distractions.
When you're a company worth so many billions (now nearly two trillion), why are you putting engineering time into something that seems to have little growth and little money? Shouldn't you re-task those engineers to projects that might be the next big thing? I know that in a certain light companies can hire more people, but the hiring pool isn't infinite and you can only grow your staff so quickly without things becoming chaotic (you want enough veteran staff members around to mentor new people who don't know what is going on with the giant systems that are created in such a large company).
To me, messaging feels like a product that everyone wants to solve because it's cool, but people haven't really figured out hoe to monetize it well. I think Facebook wants messaging to reinforce its ecosystem and fend off rivals more than anything. Apple really likes iMessage because consumers seem to be really into iMessage and it seems to create a positive feedback loop to get people to buy iPhones (I'm an iPhone user and I don't get what's so great about iMessage, but people are really passionate about it). What does Google get out of messaging? What does Signal get out of it? Signal and Telegram have both been looking for business models and they've looked into cryptocurrency, but I'd argue that neither has really found a business model.
The messaging apps that seem to have found business models are the ones that aren't general chat/text replacements, but community chat systems like Discord and Slack. Microsoft's efforts with Teams and the new Google Hangouts Chat/Meet enterprise Slack clone show that Microsoft and Google see a Slack competitor as where the money is in messaging. It's easy to get a company to give you $5-25/mo per user when they're spending $5,000-50,000 per month on that user already (not just salary, but benefits, office space, equipment, etc).
I think the real problem is that there's little money to be made in the old Gmail/GChat messaging. So, in comes some project manager that wants to make their bones solving a potentially large market in messaging and they don't have any wonderful ideas, but they're hoping that if they move enough things around and rebrand enough things they can cherry-pick some metrics and show how genius they are and why they deserve a big promotion. You don't get recognition and promotions for keeping a ship steady in calm seas. Combine that with a product that doesn't seem to meet expectations for return on engineering investment and why should Google keep investing in this?
If we look at the companies that have succeeded, they're not general messaging apps and they're usually aimed at taking advantage of an enterprise play - with a generous enough free tier that home people can play with it. I think Google didn't want to continue offering a general purpose messenger that didn't have a path to profitable growth. They also didn't want to abandon messaging. So they kept shaking things up trying to find product market fit - profitable fit, not just something that free users enjoyed without something in it for Google.