This really isn't difficult to understand (again, just read the link above, the people actually involved go into it). The systemd people believed there were technical flaws in Upstart. They talked to the person in charge of Upstart, who was amenable to their points. They looked into collaborating, but the CLA stood in the way of that.

If there was no CLA, the folks who went on to develop systemd could have decided that the benefits of collaborating with an existing project were greater than the benefits of writing systemd from scratch. And everyone involved seems to agree that that's what would have happened. Yes, as a purely technical matter, the systemd people decided starting from scratch was better than forking Upstart. But the reason it was a purely technical decision was their refusal to sign the CLA, which meant the ONLY considerations were technical. But there are other considerations that could have swung the decision the other way if not for the CLA.