That’s interesting! I can’t tell where you are posting, of course, but I wonder if it’s a Europe/US difference, given that ResearchGate is based in Germany. That EU home points to another possibility, though: ResearchGate was not subject to the flood of Elsevier DMCA takedown notices that hit Academia.edu a while back due to differences in national laws. The other possibility is that it’s a field-based thing, but I’m not sure.
As to differences — honestly, I haven’t spent any time with ResearchGate, and I don’t know the sources of their funding, so I can’t say. I’d love to know your thoughts!