Because it is loosely couped with the backend, so any type of backend rest services can be called out from the angular services, where as the spring MVCs tightly coupled compare to angular js where view also should be in JSP/JSF/ any Java supported technologies.
As this is a great single page application framework, it provides awesome next generation user experience.
As angular js is a widely used for mobile end application developments, it beats other frameworks and stay front for many reasons including this.
