Darcs is not "another", it is "the first distributed" version control system that gained some traction in open source projects. Before git usage exploded, darcs was almost exclusively used by haskel and lisp libraries.

What killed darcs, besides performance issues with darcs 1, was "one repo one branch" mode, and tedious way of maintaining long lived forks, which wasn't that convenient as what git provides.

However, I'm glad that Darcs is still actively developed and used, since it has very nice theory and model behind it, and it still has some very useful concept which are quite unique for it.