Politics.
Yeh thought so, and money.
I guess it isn't in the interests of the money in the sport to have one person holding all the belts and then either keeping them or losing them all each time a fight takes place.
I think the whole business with Fury, Joshua and Wilder is bonkers. In any sane world why would you sanction the same fight as has happened twice before (and only involves two, I think of the belts) over a fight that would unify all the belts and would clearly be a bigger fight.
I'm not really a fan of boxing at all, but if I were I think I'd find it really frustrating that a Fury/Joshua fight is being blocked by lawyers.