Why would Brown not withdraw the remark?

"Obviously the Prime Minister doesn't want to withdraw his remarks"

That was David Cameron during PMQ's earlier today.

It was one of the most fractious sessions between the two.Gordon Brown accused David Cameron of playing party politics over the death of Baby P which is plainly wasn't.

With both sides of the House calling across the chamber and the speaker being forced to intercede on the part of both party leaders,why did Brown not withdraw his remark?

Another example of how the Prime Minister seems unable to deal with anything that is not in his prepared notes