ASP.NET Core MVC is a framework that runs on top of the full .NET framework (Windows) or .NET Core (cross-platform), specifically made for building efficient HTTP services that will be able to be reached by a massive range of clients, including web browsers, mobile devices, smart TVs, web-based home automation tools, and more.
Angular is the successor of AngularJS, a world-renowned development framework born with the idea of providing the coder with the toolbox needed to build reactive and cross-platform web-based apps optimized for desktop and mobile. It features a structure-rich template approach based upon a natural, easy-to-write, and readable syntax.
Technically, these two frameworks have little or nothing in common: ASP.NET Core is mostly focused on the server-side part of the web development stack, while Angular is dedicated to cover all the client-side aspects of web applications such as UI and UX. However, they were put together here because they share a common vision--the HTTP protocol is not limited to serving web pages; it can also be used as a viable platform to build web-based APIs to effectively send and receive data. A thought that slowly made its way through the first 20 years of the World Wide Web and is now an undeniable, widely acknowledged statement and also a fundamental pillar of almost every modern web development approach.
As for the reasons behind this perspective switch, there are plenty of good reasons for that, the most important of them being related to the intrinsic characteristics of the HTTP protocol: rather simple to use, and flexible enough to match most development needs of the always-changing environment that the World Wide Web happens to be in. Not to mention how universal it has become nowadays--almost any platform that we can think of has an HTTP library, so HTTP services can reach a broad range of clients, including browsers, mobile devices, and traditional desktop applications.
