One reason; reach.
In academia reach -per se- is not a big deal, while impact is.
Reach leads to impact. You can't get impact without reach, but reach in non-scientific communities does not necessarily turn into reach in scientific communities.
At nowiki we vere approached some years ago by a
university about publishing cutting edge research in fish farming. We could not publish their work because some claimed it to be "original research". Sure it was, and it was darn good original research too. I don't think that was a single occurence, other communities has probably had similar questions.
On Wikipedia you have no means to tell what is a good research, anyway.
There are nothing that blocks Wikipedia from doing peer review. (It has implicit peer review.) What you propose for WikiJournal is to make peer review a policy. That does not in itself turn articles into good research.
You can turn your statement around and say if you can not write good reasearch on Wikipedia, then you can not write good reasearch on WikiJournal. The tools are basically the same, the only real differences are in the policies.
Actually, some of the example articles at WikiJournal are nothing more than FA, but that is another discussion.
Intent is the major difference on what WikiJournal could be compared to Wikipedia.