I would argue that these are the wrong questions. As this article, and those papers referenced by the article point out, this has to do overwhelmingly with inadequate waste disposal systems. More interesting questions for solving the issue might be:

What types of items end up in the oceans?

Are they dumped legally or illegally?

What are the reasons that certain countries don't have the infrastructure in place to dispose of waste?

How can wealthier countries do their part in supporting countries who need to develop better waste management infrastructure?

I think that it is better to ask questions that lead us directly towards solutions that are actionable. The data strongly suggests that ineffective or nonexistent waste management is the root cause, so I think we should direct our questions towards solving that issue.