ALT.NET died because a huge number of the people who were involved have left the .NET ecosystem entirely, mostly to Ruby and node.js. (For example, I wrote Ninject, which was a fairly popular OSS .NET project in its day, but I haven't written a line of C# in years.)
Ironically, now that Microsoft is much friendlier to OSS, there are dramatically fewer people interested in working on open source .NET. Don't get me wrong, it's great to see Microsoft doing this stuff. It's just about 7 years too late.
For open source .NET to regain its relevancy, it would require a unique OSS project or framework to be built on .NET and gather meaningful mindshare. To my knowledge, that hasn't happened in years.