Why was this answer deleted so quickly? The answer is apparently wrong, but why don't we give the poster enough time to acknowledge the comments and maybe fix his answer?
The poster can always edit their answer and mod-flag it to be undeleted. Deleting it sooner rather than later actually helps out the poster - it prevents piles of downvotes from accumulating, making it much more likely that the answer will be well-received after it is fixed. Since voters have no idea when an answer is edited unless they are actively watching it, many people will downvote invalid answers and never come back to take back their downvote once the answer is fixed.