Why did notable mathematician Walter Rudin have so few citations according to ResearchGate?
This answer will be a bit orthogonal to the question. But Rudin's place is due to more than the number of papers written and citations. 

According to the Math Genealogy project, Rudin produced 24 successful PhD students and has 125 "descendants". These numbers are pretty big. Some of his students were, themselves very productive. 

A professor can be a collaborator, but not an author on a pretty large number of papers, and this is pretty typical in math. 

Note also, that in mathematics it was then not normal for a professor to be listed as co-author on their student's work. I think that is still pretty standard, actually. 

So, the number of citations isn't necessarily the only measure of influence. 



But as to the question itself, mathematicians have moved on to other, newer, areas since his time. In some sense this is, or can be, due to success in solving outstanding problems, leaving only the hardest questions still open. So people work in areas in which it might be a bit easier to be "productive".