Comments are, preferentially, for teasing out information. Rather than being permanent fixtures of "this is some information" they are best used as "trial and error" or faultfinding tidbits. Real information on the problem should really be in your question. 


We do not enforce the keeping of comments like we do useful answers and a set of comments could quickly become one sided or difficult to follow. 

Imagine you were having a problem with some program not running and someone asked a question about what antivirus you used because they saw an error running certain programs with Norton. You reply "McAffee". That person decides their query is irrelevant as the problem is elsewhere and they delete it.  You are left randomly saying "McAffee" with no context and it is just confusing matters.

From the [comment privilege][1] page:

> ###What are comments?

> Comments are **temporary "Post-It" notes** left on a question or answer. They can be up-voted (but not down-voted) and flagged, but do not generate reputation. There's no revision history, and when they are deleted they're gone for good.

> ...

> ###When should I comment?

> You should submit a comment if you want to:

> - Request **clarification** from the author;
> - Leave **constructive criticism** that guides the author in improving the post;
> - Add relevant but **minor or transient information** to a post (e.g. a link to a related question, or an alert to the author that the question has been updated).

Comment can (and in many cases have) been left in place for years, but they are not first class citizens on the site. They can be deleted by their owners on a whim and information can easily be lost. If the information is important then it should be in your question where it is protected by the revision history and in relation powers of normal users.

  [1]: https://superuser.com/help/privileges/comment