I think that one factor is that staying on old tech is a risky career move both for the individual and the business. At work we have an Angular (the old one) app and while many new devs pick up some React, they definitely don't play around with ancient Angular versions for fun. So now we're stuck with a pretty big app using tech no one is interested in anymore.
Partly due to frontend framework hype, I would say. If we take traditional backend web frameworks, like lets say Django, you could simply look for Django devs and would find plenty, even though it exists for a long time. Choosing a hyped JS framework is what is the risky move, because people jump ship quickly. There are design decisions in Angular for which I would never even consider Angular for development.
When angular.js came when you needed to do something highly interactive it's competitors used to be either amature hour hipster shit frameworks or hacking stuff using jquery.
