* a genuine interest in trying new things and trying to see if they'd stick, without the baggage of established UX & customers - Allo/Duo are like this. I don't think people give the company enough credit for this.
* leadership downplaying the cost to the Google brand of shutting things down all the time. When brought up internally, execs shrug saying that we must be willing to try and see if things stick. This makes sense, but why are people particularly mad at Google for doing this? It must be for a good reason, not merely a meme.
* org silos. The org behind Google Docs / Chat has a different reason for a chat app (chat as a checkbox for enterprise office suite sales) than the one behind Google Maps (you can chat with restaurants or whatnot)
* a lack of a good "design dictator", meaning our chat apps, as with other apps, falter for lack of great UX and don't gain traction. The biggest example I can think of is how Google Chat has a loading spinner for the emoji picker - this simple thing should be lightning quick, but it took a year for someone to even prioritize it.
* faulty marketing / branding. Taking the simple, beloved "GChat", which was the dominant chat app between AIM and FB Messenger, and wringing it though "Hangouts" and "Allo/Duo" and "Chat" - that's no fun for users.
I think the lesson here is that people want a simple, hyper-fast app that gets out of their way and slowly adds nice things on top. I'd say the apps that are most fun and fast to use are Messenger and iMessage. (I have plenty of problems with both - unremovable stories on Messenger, lack of archiving chats and general slowness on iMessage).
All these are my opinions.