I use GitHub because it's simple to use, easy to learn, and pleasing to look at. Its intuitive interface makes it easy for me to find what I want. They have unlimited free private repositories, so you can keep closed-source projects on there too! Their excellent Hub command line tool gives you better integration with GitHub without even leaving the terminal.
I use Dart because it is a fast, modern language with an intuitive package manager and syntax similar to Java, while less verbose (i.e. public by default, _ in front of a variable, class, etc. to be private). Dart has an excellent asynchronous syntax making asynchronous calls such as filesystem interaction or HTTP requests simple and concise.
I use Flutter primarily because it is a fast, performant cross-platform framework. Flutter currently runs on Android and iOS, where it is compiled to native arm code. Flutter will be the primary development framework for Google's upcoming Fuchsia operating system. Official support for MacOS, Linux and Windows are coming soon.