Reason: More people -> more eyes -> more contributions -> less work for Microsoft.
FSharp Software Foundation has at least two mature organizations on GitHub which maintain F# open-source projects (https://github.com/fsharp
). There are a lot of people experienced with GitHub UX and features that (i believe) will be happy to contribute to Visual F# Compiler & Tools.
Microsoft has done this before:
As Steffen Forkmann already mentioned (http://www.navision-blog.de/blog/2014/07/05/microsoft-open-source-development-and-codeplex/
current version of CodePlex has some issues in user experience and gaps in functionality that slows down progress of Visual F# Compiler & Tools.
Small community voting shows that people are positive about migration to GitHub: