With what celebrity culture is and how much focus there is on celebrity couples, you can definitely see how Jada Pinkett Smith might be apprehensive about starring in Ali opposite her husband.
This might be true for any movie, but I imagine it was especially true for a movie like Ali that is about real people.
There is a desire on the part of the actors, in this case Jada Pinkett Smith, to be respectful and do justice to that story and those people, and part of that is keeping the focus on the characters and not the stars playing them.
The thought being that some audiences might see the couple that appears in tabloids and on red carpets, and thus not get as immersed in the film as they otherwise would.