Originally Posted by
Sazzy
It's not the pureness that matters. The drivers for nexus machines are published openly by google quickly after each release. That makes it a very easy device to create custom roms for. Manu's like Samsung only publish the bare minimum of sources, making it much harder to create a stable rom for, since the kernels for these devices are often hacked together, rather than a pure original kernel.
Besides that, google also delivers updates long after others and much quicker.