This is easy to answer. Because C is ipso facto faster for this sort of thing. And one of the goals of Perl 6 is to make sure we don't continue to lose out in the performance stakes.

Not only that, but who would write it? Perl people tend to know C as a second language. So if you want contributions you need to code in a language people are comfortable in. At the p5p meeting just before Perl 6 was decided on as the next step, Chip Salzenburg (sp?) asked what languages people would be most comfortable hacking Perl 6 out in. C was the overwhelmingly most popular choice.