Might have something to do with the fact that coLinux is almost as old as Linux itself. I was surprised to see it on the front page of HN, it was an iffy solution back when it was a semi-viable option. Now that hardware-assisted VMs with very low overhead have been commonplace for a decade, it's pretty much completely obsolete.

Edit: I guess development on it started in 2004, it was only a few years later that hardware-assisted virtualization started becoming relatively mainstream.