Not all challenges require leaderboards
The reason leaderboards are used at all is to find answers based on byte count, based on language, based on user, etc.. Finding answers is not a big problem on challenges that only have a page's worth of answers or less.
However, challenges with over 30 answers can get clunky to search for answers in. In that case, when the challenge was posted does not matter as either way, a leaderboard is useful.
Now, I am not saying we need to put a leaderboard on every challenge stat. If you're going to edit leaderboards in, be considerate of how you're going to be bumping it to the home page. Don't bump dozens of old challenges just for this purpose. A good way of going about this is putting a leaderboard on a challenge if you are about to answer it, in which case it will be useful to you and others as they come in later.
The "popularity" of a challenge is also a factor. Note that this is slightly subjective. Before you stick a leaderboard on a challenge, think about whether the challenge is something that will be searched often.
There's a possibility that every challenger could include a leaderboard. While I do not currently see any disadvantages to this, I'm not sure if the community would agree.