Publication of review's comments on scientific papers? Personally I believe publication process is not optimized at all and is too wasteful. There are numerous problems associated with it, however, here I just point out one of those. 
Due to formal presentation of papers, usually reading a scientific paper especially in field of science and engineering is time consuming. There are some points in the paper that only a specialist of that very field would notice. This is why the journal editors assign reviewers to judge the paper quality. Usually these expert reviewers would see insightful points on the paper, which would take a newbie months to notice. 
Having said that 


I wonder why the publication does not provide review comments alongside the paper? does it harm anything?
Would be OK if we ask the editor for the comments of an specific paper? if yes, how?


I think it would be a good idea for publishers to publish reviewer comments of papers as well, they can even make more money and reading and understanding papers would be easier.
It would probably harm the reviewing process itself as it stands at the moment. Many reviewers only agree to review, because they can be anonymous and their comments are confidential. I therefore do not believe, that any editor of a traditional journal would give you access to the reviewer comments on a paper, however you ask.

Having said that, there are some recent developments in some journals, which I appreciate a lot, and which go in that direction:


Copernicus Publications is promoting an open review process. All manuscripts are published in a discussions version (pre-review), and reviewer comments are published and archived together with the article (and can always be referred to from the finally published paper).
Frontiers is promoting non-anaonymous reviews. The reviewers are mentioned by name on the title page of each article. In my opinion, this increases the quality and objectivity of reviews a lot.
There is always the possibility to publish additional reviews after publication on post-publication peer review sites, like PubPeer. Those can help you to find errors in already published articles, which may have evaded the original manuscript reviewers.