Lack of backward compatibility: The change in technology and complexity needed to be supported by applications resulted in tedious and inconsistent implementations with first generation of JS framework. As a result newer versions are redesigning framework by starting from scratch. For example: AngularJS 2 is drastic change from earlier version of AngularJS. AngularJS 2 is not just upgrade of older version but rewrite of framework and design approach. For example: AngularJS 2 is focuses on component based approach or Angular2 uses Typescript which introduces concepts like Class, generics and lambda expressions.
Scarcity of skill set & support for older versions: With quicker rollouts of newer versions of framework is resulting in lack of community support for older versions, and scarcity of skill set for older versions. Also lack of support for older version can hurt security and robustness.
Governance will be key to manage this situation.
Keep right balance between functional supported and maintainability