I suspect everyone is fixated on it for two reasons. First, in general, it's easy to measure sensor performance and plot the data or reduce the data to a single 'score'. Second, in particular on CR forums, it's an area where Canon is not leading, and some people like to create a stir by beating on that horse (we sometimes call them trolls...).
You are correct as usual – EXCEPT – is it really "an area where Canon is not leading?" Or is this troll-fueled internet forum conventional wisdom?
How is Canon behind in its full frame sensors? Canon has taken a different path in that Canon has decided to go for high ISO performance instead of higher megapixels (which, by the way is what the conventional internet forum wisdom was clamoring for prior to the release of the 5DIII). But, a different path isn't falling behind, it's a different path and one, frankly, that I prefer. Truth is, I don't really need either a lot more megapixels or a lot higher ISO, but if I'm going to get one, I'll take higher ISO.
As for APS-C, Canon's flagship camera still uses a sensor that is better and newer than Nikon's flagship APS-C, so how is that falling behind? Nikon has put some interesting higher resolution sensors in their consumer models and maybe Canon is dragging their feet a bit in releasing their own versions, but it's hardly the end of the world. The higher megapixels do nothing for me and frankly the high ISO performance of the sensors Nikon is using is unimpressive, especially when you consider they are competing against Canon's nearly four-year-old sensor.
But, even if one believes the competitors have moved ahead slightly, how is that bad? Products only improve when competitors offer something better or different. Canon wants my money and when they release the next product I want, they will get it. In the meantime, I'm not going to take it personally and act like the sky is falling.