When Silverlight 1 came out in 2007, there were three competitors for it:
- ActiveX which was a horrible 90s idea and is unable to function in a world where you can't trust people not to try to build exploits
- Java which was so bad at doing what it was supposed to do that it went from almost 100% market share to almost 0% with the rise of Flash.
- Flash which did the job it was supposed to do but had horrible development tools and literally hundreds of security problems since then due to shoddy product quality
Microsoft created Silverlight to solve these shortcomings and they did a pretty good job at it. Programming web code in Visual Studio is a leaps better than Flash and the Netflix probably saved millions by not wasting their developers' time with the horrible Flash UI and code oddities.
Only now, four years later, is HTML5 beginning to come to a point where it can be a proper tool to do what you used to use one of the above plugins for.
And by the way, IT changes fast in general, no developer can honestly expect to code in the same language from college to retirement. HTML5 - and the languages that you actually write code in like JQuery - are in an extreme prototype state right now, going to change radically several times in the next years before people figure out that they completely screwed up some important paradigms and start parts of the standard from scratch for HTML6. Everyone will have to keep relearning their languages if they want to stay current.