Why, it depends on the application requirements. Angular 2 is going to yield a heavily structured, opinionated framework to develop your app. React (or Preact) takes care of only the view layer. You can look at this as either freeing you to choose the rest of your stack (state management, REST calls, routing, etc.) or forcing you to construct all these components yourself.
In my experience there have been many reasons for going with a micro-service based application constructed of many smaller pieces because it often lends itself to quick iteration and piecemeal patches for bugs as opposed to monolithic releases from your favorite library. The flip side to this is of course little-to-no long term support, which will require regular maintenance.
There are Typescript bindings you'll need to worry about with Angular that presents a whole other layer on top of your app that won't be a concern in React. And there are already whole UI stack templates out there ready to use for React (create-react-app) and boilerplates for Angular. It really comes down to what will best suit the needs of your application and the developers, both present and future.
There is also a sort of middle-of-the-road candidate gaining steam called Vue.js. You may want to take a look at it.
Without more info on what you're building that's the best I can do.