Since publication involves two major components, first, submission of papers and second, the judgment as to whether the article should be accepted based in part on peer review, it is clear that the reason for the low number of publications from developing countries is either a reduced number submitted, or a higher rejection rate. Clearly both the submission rate and acceptance rate for developing countries are considerably lower than what is expected. As for instance a scientist from a developed country, adjusted for the proportion of scientists, is over 5 times more likely to submit to the BMJ than from a developing country and also articles from developed countries are more than 2 times as likely to have their articles accepted in the Annals of Internal Medicine. 

It is not so easy to clarify the causal network of why this is occurring, as some of them are related to the sociopolitical reasons that why most countries are still developing but one possibility, which comes to your mind easily is the lack of representation of developing country scientists on the editorial boards and reviewers. Also, we should never forget that scientists not only receive no income from the journals but often have to pay them page charges under the same contracts by which they surrender their copyrights. Most of the time it is not possible for researchers, for instance in Iran, to submit their articles to the western journals as they would be charged up to their monthly salary!