That had everything to do with Canonical entanglement. Canonical would only accept patches to upstart with a contributor agreement allowing Canonical relicensing rights. This made it very difficult for outside contributors to work on upstart (especially redhat funded ones) and that led to resources being made available for investigating what would become systemd and friends.

RHEL 6 (and CentOS 6 by extension) use Upstart: https://en.wikipedia.org/wiki/Upstart#Adoption

Not that I disagree with your comments on Red Hat, but let's get our facts right.

RHEL6 starts upstarts, but doesn't make use of any of the functionality. Everything is sysvinit scripts and zero (upstart) jobs. I was administrating a few RHEL6 machines and had to read elsewhere about the change to even notice it.

The person you responded to is correct though; a large factor in starting systemd was the CLA of Upstart. Above even highlights this; you cannot really say that RHEL6 used upstart in any way.

I guess it depends on how you look at it. It uses upstart to start all the sysvinit scripts, no? I know you can create upstart jobs and they'll get started, I've done that. But you're right, it doesn't make "real" use of upstart, in a sense.