Would it be fair to summarize that being more rationalist doesn't much increase your chance of choosing poly on your own, but strongly increases your chance of being convinced to become poly by other rationalists?
People usually learn from each other. For example, I haven't invented math nor programming; I just learned about them from others, liked that, and became good at it. Not sure what exactly that implies, but I definitely don't see it as an argument against math or programming.
If you are in the rationalist community, there is a chance your partner is too, so you both get introduced to the idea. That removes a potential obstacle of "how to explain polyamory to your current partner without ruining the relationship".
(Disclaimer: This is not an unmotivated reasoning. I was trying to generate answers to what you could possibly be missing.)