Lack of available reviews on scientific papers
Usually the issues pointed out in a review have been corrected by the time the manuscript has been published, so old reviews wouldn't really be relevant to the online version. 

Critiques of published articles are sometimes prepared, if one thinks the issue is significant enough; they're usually published as "Comments" or "Responses" to the article in question. However, since they're normally also peer-reviewed, they won't simply be appended to the article, but instead have their own independent status.

As for the reason why more articles don't have reviews and commentary associated with them—it takes a lot of time to do so, and that's time that can be used to make progress in one's own research. And there isn't much incentive right now for such efforts.