Because it’s built and supported by Google and it would be a dumb career move to tie your horse to anything Google creates. It will probably be abandoned.
Google has the attention span of a crack addled flea
Google has the attention span of a crack addled flea
Google is behind Android, and there's a ton of android developers. It took off.
Google built Golang, and there's a ton of Go developers. It took off.
Google built Angular and Dart, and they each had much less success and didn't take off.
The "Dart isn't popular because it's backed by google" theory doesn't seem sufficient based on the above.
A more reasonable theory would be that the answer to "Why isn't X popular" is that the vast majority of things _aren't_ popular. Why isn't D-lang popular? Why isn't Crystal popular? Why didn't Elm take off?
None of those are made by google, but it turns out that building a user-base of a programming language is hard no matter what.
There is also Google Cloud IOT Core, Conversation Actions, Android Things, Google Chatbase, and even the original Angular was abandoned by a completely incompatible framework.
That has no relation to whether something is B2C or B2B. Stadia is a consumer product, ie for consumers.
You have a point there about those other ones, but honestly, those are so few and far between that I'm sure you had to search somewhat carefully to find them. I highly doubt many devs were actually using those (besides Angular, but it was replaced with 2+ because React came onto the scene, it didn't get deprecated with no replacement).