It's spam prevention. Everyone is actively reading questions and answers so spam in those types of posts gets reported immediately and deleted. Comments are u

It's spam prevention. Everyone is actively reading questions and answers so spam in those types of posts gets reported immediately and deleted. Comments are unobtrusive (new comments on a post don't bump it back up to the top of the active queue like new answers do) so fewer people are looking at them. If anyone at all could comment at any time, spammers would have a zero-barrier point of entry. Setting the limit at 50 reputation gives people a very low hurdle to get over before they can comment.