There are a few more Ruby ones around. I have a list at [0] but even that one is outdated, as it does not include Ace or Stasis yet… and I’m not familiar enough with Python/Perl/JavaScript/etc to know the state of site generators there.
Writing site generators is easy and homegrown site generators will fit the author’s need best, so there you have the reason why there are so many. Mind you that quite a few are no longer actively maintained. For example, StaticMatic and Webby have recently both been discontinued (see [1] and [2]) and their authors recommend MiddleMan and nanoc, respectively.