What is the precise reasoning behind this practice? For instance, why would this encourage "better" participation or lead to more meaningful responses than restricting all contribution to other users' topics until reaching 50 rep; or why not restrict answering questions until such rep is attained?
Were trials/studies done on this during development of the SE platform?
Stack Exchange is not for comments. It is for Questions and Answers. Comments are intended for asking for clarification, and are usually temporary - deleted once clarifications/updates are made. So we don't want to encourage comments.
This has been answered on an SE-wide meta post. Core from that post:
Comments If we allowed all users to write comments, that would likely read to a lot of people saying things like
Thanks!
And that's not what comments are for. Thus, we limit the people who can do that to only those which we trust, and the best way to quantify trust on the Stack Exchange network is through reputation.
On top of that, a simple and annoying reason for all of these is that they avoid spam. It's easier to catch a spam question or answer than something minor in a comment, and chances of spamming decrease with the reputation of a user.
Low-rep users might also jump in comments and say offensive things, so it's nice to avoid that.
And yes, various trials and studies have been done over the years - SE are very good at involving the community in updating features and functionality.
