Nobody is stating there is a single factor. But it appears to be the largest that we can plausibly put a finger on. When you see the same correlations across different countries, states, and local jurisdictions that did not have identical drug policies, immigration/migration patterns, policing strategies etc. etc. etc, it becomes convincing. The three studies in this article provide even more evidence, using clever experimental designs to control for as many of these other variables as they can. Did you read the article? Do you have specific comments on how those studies are or are not contributing to our knowledge?