I hear that a lot, but why do folks say that? It has just become a mature and stable UI platform suitable for long-term development. What's the alternative?
Because it was at some point IIRC,they didn't have a team for it internaly. This was back in the silverlight shutdown, html5/winrt win8 drama. Silverlight actually died, WPF got revived. This is just going on memory, and I wasn't really paying attention to it anyway.
IronPython and IronRuby are still useful for doing .NET things in a REPL or scripting, though Roslyn's support for C# (and VB) REPL/scripting and F# have lessened the amount of time I spend with IronPython.
(I did recently hear IronPython rebooted and is trying to build out a full CPython 3.5+ compatible version, which was exciting to hear.)
I haven't had the chance to play with Roslyn yet, though. Seems promising.