Could it be that the Rats have concluded that legal immigrants would be less dependent and loyal to the progressive socialist causes, and less of an adverse impact on the US economy?
Yes
You don't seem to understand the process. There are Congressional limits on the numbers of immigrants admitted under various categories. The Dems would love to see them all admitted as soon as possible since they vote two to one Democrat. 87% of legal immigrants are minorities as defined by the USG. Immigrants and minorities vote two to one Dem.
Immigrants, legal and illegal, use the welfare system to a far greater extent than the native born. Immigrants are natural Dem constituents. They favor Big Government.