Reason #1 - Cross platform
ASP.NET Core is cross-platform. That means you can use Windows, MacOS or Linux to build, run, deploy and host your application built using ASP.NET Core. Both Visual Studio Code and Visual Studio Community are available cross-platform and are used by many developers as their day to day development tool.
Your ASP.NET Core API project can be easily deployed to multiple platforms to host it inside IIS, Nginx, Apache, Docker or self-hosted model.
Reason #2 - In-built Environment Targeting
Reason #3 - High Performance
ASP.NET Core has modular and lightweight HTTP request pipeline that makes it high-performance. Its built to provide fully flexible HTTP request pipeline customization using its middleware mechanism and still uses the simplest and smallest possible HTTP request pipeline by default to serve any request that is coming through.
Reason #4 - MVC & Testable
Reason #5 - Data Access, ORM & Entity Framework
Entity Framework for ASP.NET Core makes it easy to access data from any database using Entity Framework. Apart from Microsoft Sql Server, it also supports MongoDb, MySql and PostgreSql which makes it a great choice. Regardless of the database server that you use, Entity Framework development is almost unified to access any database that you are using.
Reason #6 - Security
ASP.NET Core community has very good open source projects like Identity Server, jwt token authentication and ASP.NET Identity which makes it easy for your API to integrate any kind of authentication like forms authentication, social authentication, token authentication, OAuth based authentication etc.
Reason #7 - RESTful
ASP.NET Core has in build support to follow best practices while building RESTful APIs. With ASP.NET Core Web API, Building RESTful clean urls, API versioning, localization and globalization etc is already resolved for you and you just need to focus on building business specific RESTful APIs.
Reason #8 - Swagger & Open API
ASP.NET Core supports integration of Swagger & Open API that makes it easy to automatically document your API end points that is huge help for any developer who is integrating your API to their application.