Because their cores solve the same problems, and it would be overkill to pull in both projects for views or models. Also, I don't think backbone.js models solve any problems in an Angular application.
From what I've seen on Angular people are really hyped about extending html and two way binding but when it comes to communication with the server they just wrap arbitrary ajax call done via $http or $resource in Angular service or factory or provider and call it a day. After seeing what backbone does, this seems to me to be as reckless as manipulating the DOM using jQuery.