[mwcampbell]

So why did you say C# and C+= are a much better fit for XAML? Because they're the languages that Microsoft officially supports? Any other reasons?

[Aleman360]

Official support, documentation, perf (presumably), debugging, and tooling support would be my main concerns.

Technically, XAML makes heavy use of C#'s events and properties (which C++\Cx adds to C++), which don't exist in plain old JavaScript. Also, both C# and C++\Cx support "await" natively, which is extremely nice for WinRT's async-heavy API's.