Several reasons, -Silicon efficiency (QE) peaks in the green -Green spectral response curve is close to the luminance curve humans see, like you said. -Twice the pixels to increase the effective resolution in the green/luminance channel, color channels in YUV contribute almost no details.

Why is YUV or other luminance-chrominance color spaces important for a RGB input? Because many processing steps and encoders, work in YUV colorspaces. This wasn't really covered in the article.