Why a Mayweather vs. McGregor type fight never happened historically, and why it can happen now

The idea of the biggest boxing star in the world facing a world champion from another combat sport has been talked about since the beginning of time, and now it may actually happen. But the irony is that as successful as UFC is, they are doing something that no other company in the past ever agreed to do: sacrifice their top draw for a one-night payoff. Dana White's latest revelation, that a proposed Floyd Mayweather Jr. vs. Conor McGregor fight will probably happen, besides being a quick about-