Among others: because of completely arbitrary and I'd dare to say weird rules. Programs are not meant to be written in idiomatic language. They are meant to resemble the C implementation. In many cases you can see that the program representing the language in the comparison is neither the fastest nor the simplest of the submissions.
You've made a bunch of claims without showing any evidence to support them. Please point to specific examples we can all look at on the website which show -- "Programs are not meant to be..." -- which show -- "They are meant to resemble the C implementation." etc
I'm not going to back up these claims in any other way than pointing the "interesting alternative" programs that appear in some comparisons, which tend to be faster than the chosen versions. For example here:
http://shootout.alioth.debian.org/u64q/benchmark.php?test=fa...
Both Lisp and Java alternatives are faster than the "fastest" solution (written in Fortran). Finding any more support for the years-old opinion I voiced seems too much work to be worth it.
So we can immediately dismiss your "Programs are not meant to be written in idiomatic language. They are meant to resemble the C implementation." claims as baseless.
Yes, you can. Just one last mention: my opinion was formed in the times I used Perl and looked around the actual implementations of these. But that was years ago. It was reinforced quite recently by someone whining (probably on HN) about the same things. Apart from these anecdotes, I have no basis for these words.