It's because a the database isn't completely agnosticly load-balanced, and a single story is stored/processed with more co-locality than a random sample of, say

It's because a the database isn't completely agnosticly load-balanced, and a single story is stored/processed with more co-locality than a random sample of, say, 10000 randomly chosen comments. It's easier to cache and serve a fixed piece of content than to constantly insert and re-compress it (which kills cached versions too)