Inertia, mostly. If you want your research visible and you want mad street cred for tenure, you publish in the biggest and baddest journals. The biggest and baddest journals are the biggest and baddest because they've been around since the days when scientists would go the university library to read papers.
I agree with inertia, but I want to add a few more details from experience that could shed a bit more light onto it:
* There are not that many open journals around (see e.g. [1], probably slightly outdated); plus the throughput of a journal is generally low (the linked journal ToC has published 20 articles in 2015).
* Graduate students often want to maximize the impact of the journal they're submitting it to simply because the work itself is say of medium importance to the field.
* The review process takes a lot of time, it can even take more than 100 days. This encourages the researcher into trying as few journals as possible, which benefits the more impacted, more numerous closed journals.
* At least at my university neither the administration nor the school library promote open journals in any significant way. Which is strange, seeing as they are the ones paying the journal subscription fees.
Inertia, but also it is convenient for funding commities and electoral bodies. It removes the burden of having to review the actual scientists, since they can rely on "where they publish". This is just as flawed as it sounds, and IMHO has led to a lot of dubious science in the life sciences.
