The UFC will not allow a Boxing bout between their biggest PPV drawcard and Floyd Mayweather. At the moment they can say McGregor is the baddest S.O.B. on the planet and MMA is the baddest sport (which it is).
If Mayweather destroys McGregor IT HARMS THE UFC's PPV VALUE AND MAY HELP RESSURECT BOXING AS A PPV DRAW CARD. (Boxing and Mayweather would love a Boxing bout between Mayweather and McGregor, it helps Boxing and harms the UFC).
It harms the UFC financially. (But they will keep talking it up, but it is bullshit, just to promote the UFC and McGregor).