Because it could be disastrous for the life of many boxers who are obtaining average purses among other things, imagine, you bust your ass for 3 months of your life and you can't bring bread on the table for your family. Imagine how a very good boxer but unfortunate enough to get 2 loses back to back and nothing to feed his family. Boxing is already a demanding and hard dedication work, if you make it possible to have nothing in return of a hard work, you're just killing the sport and the drift to it.
To give you an example, It's like if you work in, let's say, a cardboard factory. It is shit because it is a factory job but it pays quite good per hour. One day the manager of the shop inform you of a new rules: The team that produces more because they work harder earns the pay of both team (night and day shift) and the others nothing. Would you want to work a full week, busting your ass to earn nothing in return? It would be a bit cruel and insane, especially because you worked really hard for that. NOt being the best man on a day doesn,t mean you do not deserve some kuddos for the work ethics.