The same reason craigslist classified haven't been surpassed by a more modern system -- it is much easier for everyone to live with the shortcomings (which really aren't that bad) than it would be to recreate an equivalently large, diverse and vibrant community on another site.

Especially since any focused site that came up to do this would almost certainly put way more barriers to entry into the community than reddit (which still allows you to create a new account virtually instantly, with no email verification) has.