When a measure becomes a target, it ceases to be a good measure. If an article/research quality is assessed by if it appear on pubmed, then appearing on pubmed becomes target for all research. And thus appearing on pubmed stops from being a good indicator.

You can ask for specific ways this can happen. But that would be point less. Human behavior and incentives work in ways that are beyond specifics.

Everyone here probably knows this already, but forgets it when it comes to medical domain.