Supposedly for petty personal reasons:

The woman who controls the company, Shari Redstone, snatched defeat from the jaws of victory last week as she scuttled a planned merger with David Ellison's Skydance Media.

Redstone had spent six months negotiating a complicated deal that would have given control of Paramount to Ellison and RedBird Capital, only to call it off as it neared the finish line.

The chief reason for her decision: Her reluctance to let go of a family heirloom she fought very hard to get.

The fandom doesn't want to talk about it, but the Avatar franchise is in trouble.