The fact of the matter is, there is a greater demand for US citizenship than "supply" (amounts to political will, really).

It makes sense to prioritize for the most productive/least diseased/least criminal/etc when possible to discriminate with reasonable certainty. Of course, this is not to imply that immigrants are neither unproductive, nor unhealthy, nor unlawful.

While rags-to-riches stories are great, riches-to-riches probably has a better expected outcome.