I think there were several moving parts that came together:
That last point may have actually been Biden's undoing. By conjoining CNN's need to be seen by the widest audience possible in order to showcase their new commitment to being "fair and balanced" with Biden's need to be seen as better than President Trump, both greatly miscalculated the impacts of their needs and resulting decisions.
CNN succeeded in meeting theirs, Biden failed at meeting his.
The Biden team failed to realize how CNN was going to use the Trump-Biden debate as a marketing tool to maximize total global viewership of the new and improved CNN product. What they got was a maximized total global viewership of the degraded and lied-about Democrat/Biden brand product.
The irony is that if CNN had kept exclusive rights to be the sole broadcaster of the debate, not as many people would have seen it directly and the LAAP-dog media might have been better able to contain the imagery and "cheap fake" the clips that do get shown to keep the illusion of a vibrant and "in-charge" Biden going.
-PJ
C-SPAN also simulcasted the debate. I wonder if all those who did, had to pay a fee?