Silverlight was a subset of WPF really to compete with Flash. In 2005-6 both were huge pushes and competitive, Microsoft with Silverlight, Flash with AS3/Flex. Part of this was due to Flash video/Silverlight video that took over the world in 2005-6 with youtube and more. This was just before mobile killed Flash and thus Silverlight. Silverlight never got off the ground except for video really and will not come back. Microsoft had another Flash competitor when Flash3/4 was out called Liquid Motion, same fate. When Microsoft abandons a platform you better as well.

All of them were awesome vector based rendering platforms that had great data/web integration. They were also complete packages but thrived when plugins thrived until mobile and Chrome killed plugins.

Flash/Silverlight still have some advantages over html5/webgl/canvas/svg (much of if by Khronos, funded by Apple initially) but those have essentially taken their place and WebAssembly that came from Emscripten/asm.js from Mozilla will eventually be another option.

I do miss the vector nature of Flash/Silverlight but both are gone, especially the latter. There are better options like Haxe, OpenFL/lime, Adobe Animate (Flash successor in web standards) if people still want to use Flash like platforms.