It may be less flavour of the month, but that seems par for the course with JS frameworks.
I guess on Jeremy Ashkenas twitter feed he called Backbone "finished" in terms of API and feature set after the 1.0 version. When you look at the contributions graph, its practically non-existent: https://github.com/jashkenas/backbone/graphs/contributors
Add in the Google trends graph compared to Angular and React: https://www.google.com/trends/explore?q=backbone%20js,angula...
Then when you look at the other libraries in the ecosystem, its like a ghost town.
backbone.forms - Open Issues: 172, Issues closed in 2016: 14, Last Release: Jan 21, 2016
backbone.localstorage - Open Issues: 45, Issues closed in 2016: 1, Last Release: Jan 22 2015
backbone.paginator - Open Issues: 20, Issues closed in 2016: 7+, Last Release: Feb 3 2016
backbone.validation -Open Issues: 79, Issues closed in 2016: 1, Last Release: May 8 2015
So not only is interest waning, the creator has called the library "finished" with not much more in terms of features coming and the ecosystem surrounding Backbone has all but dried up. I would be hard pressed to say it still has a bright future.
Maintained and supported. How many JS libraries have developers jumped on, only to see them die a few years later? Backbone JS is the first that comes to mind.
Everybody was using it, but since then, it's pretty much fallen by the wayside and there is still a ton of Backbone code out there that needs maintaining until those projects can transition to something else.
Having a larger enterprise company maintain a library like Typescript when you know its going to be stable and maintained for some time as opposed to a smaller library where the creator just decides to pack it up and leave everybody hanging just seems like a smarter move to me.