It's no sense to can't comment without gaining reputation. I've to comment that it's obligatory to say/write salawat when mentioning final Prophet Muhammad ﷺ name
Users can't post comments on other people's posts unless they have 50 reputation. This is a network-wide policy that is explained e.g. here: Why do I need 50 reputation to comment? What can I do instead?
Comments do not bump a thread to the main page and they also do not have review queues or voting etc. Hence it is difficult for the community to moderate them. If new users were allowed to post comments it would be exploited by spammers.