Wholeheartedly agree there's a setting factor to it as well... There have been games I've ran where nobody got the setting (and that's on me as a GM) even if they got their character.If I'm being honest, I've found that it's not the number of rules that inspire roleplaying as much as how much players get get out of the rules. I didn't get more RP out of my group with 5E, which has considerably fewer rules tacked on to its skill system. In fact, I created a set of "skill actions/uses" for my players in the first month because they were hesitant to use skills because they weren't sure what they could actually do with them.
For whatever reason, I think the biggest inhibitor I've seen to RP is just the fantasy landscape sometimes. When I run an FFG Star Wars game, people get into it deep because I think they more deeply understand the universe and can engage with it more. Similarly, when we play Tales from the Loop I think many of them connect with the being a kid and the idea of the 80's that they can engage with the setting more easily.
Groups will experience different (well, to be redundant) experiences that are more complex than any singular factor though so all of these considerations have some weight.