While Haskell is ranked ahead of eg scala or erlang in total repos on github, the majority of larger/active projects in Haskell are still in darcs repos, on places like code.haskell.org.

Historical note: the Haskell dev world jumped on distributed vcs a few years before git appeared, via darcs, and that early move has resulted in the situation today, where it's mostly only the younger projects in git.

And just as an example, I have around 200 darcs repos, including big projects like xmonad, yi or bytestring, that might never be on github, since they're in darcs on c.h.o.

The Haskell world is really an anomaly in this regard.

Not really, same applies to Smalltalk, and I'd guess Lisp as well.