Since the sites are open to anyone without registration, we need to restrict some functionality to users that have shown they understand how the site works and can write good questions and answers (instead of the aimless ranting regularly found in some other, more general, message boards).
Very new users that have no idea how the site works could abuse the ability to comment on questions to bother answerers with completely different problems. Some of us even have received emails like this, because our personal web site address was available from the user profile. Commenting does not lead to the topic being commented on being pushed to the front page, where others can review the new content and flag or down vote inappropriate or off-topic content. The ability to commenting anywhere is therefore restricted.
Only once you've shown that you understand how the site works and have some experience with it, further privileges (such as commenting, or voting to close, or editing all posts on the site) will be available, unlocked in the order of potential harm you can cause.
That this restricts the ability of new users to post e.g. answers to troubleshooting questions is unfortunate.
Commenting is open to newly registered users that have shown they understand how the Stack Exchange sites work on any of its sites (by gathering a few hundred reputation on another site) through a +100 reputation bonus. So it really is a privilege that all users should have, and that's why the requirement to get it is low, compared to other privileges.