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I’m not overly familiar with any of the new JS frameworks, but I’m not sure the answer is any more complicated than “They are both popular frameworks”. If you mean “Why use Django at all?” The answer is that you require an extremely robust backend when a lot of your logic is client-side (and thus easy for the client to manipulate). Django makes it quick and easy to set up a REST framework than you can hook into with angularjs and allows you to keep certain logic hidden from the client.
To add: People like AngularJS and other JS frameworks because they can provide a cleaner and more responsive user experience (form submission errors being displayed without the need for a form submission, as an example). A complaint I’ve heard before is that some people feel that Django does not do enough to separate the business from the display logic (something I personally disagree with, but that’s irrelevant), so it’s possible those people like the deeper separation you get by using AngularJS for all your display logic.