Cause a server with these languages has high overhead in memory and CPU to serve requests thus having lower throughput per server and thus needing more servers to sustain certain site-wide SLA and thus causing more money overall.
What I'm really trying to say here is... prove it.
Actually, you do! :) http://code.reddit.com
But to help you out, I'll tell you that a good chunk of the expensive loops are written in C.
This also calls into question the original assumption, that posters believed Reddit had far too many wasteful servers to handle their service, and that it was because they use a dynamic language.