Question for Cricket fans.

Although I do follow Cricket, I wouldn't call myself a connoisseur or even a big fan. I would therefore like to ask any cricket fan who might stumble across this blog the following question: Why didn't Australia ask England to follow on, after England was all out 445 runs behind Australia?

I accept that there was no real risk of Australia losing, but it seemed to me like the perfect situation to "invite" the opposition to follow on. That way Australia would be ensured to have enough time to bowl England out, even if a freak of nature stopped play for a half or whole days play.