Why is nitrile rubber good?

I'm just going to go ahead and guess density. Because it looks like first you compare them at the same thickness, and there PTFE is better centimeter-for-centimeter, but when you match weights rubber is better gram-for-gram.

Well, I'd say the most obvious explanation is that the latter has a lower density, so 1cm of rubber just weighs less than 1cm of PTFE.

Which is also the same reason Silica Aerogel was the undisputed king of anti-laser armor back when it was bugged. Even back then, aramid fiber was better for the same thickness. But silica aerogel was far less dense, so you could take a much thicker layer at a lower weight and cost. It was able to make up the difference in volume.