Why would a mainstream news outlet do a story based on the assertion by a private company that its Super Bowl ad got rejected?

The network refuses to comment on claims it has rejected an ad, so all you've got is a company with an immense self-interest in lying about it. You know, I'm not even going to link to the news website I'm talking about, because the answer to the question in the post title is that the news site is whoring for traffic. The private company would like to promote its product without forking out the millions needed to buy air time on the Super Bowl, and the news site would like to be the go-to place for ad, which isn't clever enough to go viral without the boost of the banned-from-the-Super-Bowl claim. Moreover, the product is shoddy and not worth talking about. Don't be a sucker.