For the same reasons he listed above, eg you want to use an ffi etc. This is not a temporary issue with c++ as you imply, there are really hard issues about the c++ programming model, eg exceptions and classes, being really hard to interface with other languages that work differently.

The question is not why Haskell not C, sure if you can use Haskell why not, but for most use cases you cannot. But most languages can be used instead of c++, Haskell or Java or whatever, with many advantages.