Younger less known journals let authors keep the copyright to their article. This allows the author(s) to post it on Researchgate.and be available to disseminate to the whole world. Whereas established journals retain the copyright and do not allow you to post on Researchgate. The article in the established journal is only available to those who subscribe to the journal and accessible only thru the libraries that subscribe. The number of subscribers including libraries may be limited. That and it is quick and easy to post on Researchgate versus waiting for reviews from a journal, revise and resubmit, and then wait for the publication date.