You need to be able to modify comments for some amount of time to correct typos, but if you can modify them for too long you can rewrite history. The current time limits are arbitrary but seem roughly correct, judging from the fact that users complain roughly equally about missing typos till it's too late, and people they're arguing with rewriting their comments.
I think the main reason is to keep the context of a discussion in a thread.<p>Turning your question around: Why would you want to modify a comment after one hour?<p>Read your comment before posting and be happy with it at that time of posting. If you come to new insights later then post another comment. There's no shame in that.
It's to make sure that history does not get 'revised'<p>As it is there is already plenty of that going on.<p>I'd be fine with locking a posting or only amending it after it gets 1 comment or upvote (or downvote).