Stack Exchange doesn't allow you to comment on questions unless you have 50 reputation points. How do you earn those? By asking questions.
You get points by asking questions or answering them1. If you've got something to share, you can – and are actively encouraged – to do so. But that sharing should focus on questions and answers, not comments.
What you have to remember (or realise) about the StackExchange sites is that unlike many other websites, they aren't "forums" or "message boards" where someone posts something (hopefully) interesting and all-and-sundry chip in with their views. Instead, its "heart and sole" is to be a collection of questions and answers that (a) hopefully help the original asker, but (b) almost more importantly will help others in the future.
Personally, I'm not sure whether 50 is the right limit, but – as I understand it – one of the main reasons for their being a limit is to stop new users arriving here in "message-board mode" and flooding questions with comments. Once they've been here a bit and "learnt the ropes", then they get the ability to comment.
The two main downsides to the limit is that (a) some people are dissuaded from hanging around long-enough to get the initial 50 points (although it's not too onerous a task), and (b) people post comments as answers (and then get even more dissuaded because of the backlash that follows).
1 There are other ways to earn points, such as by editing, but asking and answering are probably the main ones.