Another potential reason is the decline in popularity of the sport. This is possibly linked to the move away from free-to-air TV for the last number of decades. Of course, real-time TV is also in decline as viewers switch to on-demand and online offerings. It now seems that both boxing and real-time TV have a common interest.
Not all US real-time TV is in decline. Last week the Financial Times carried an article (here) explaining how Spanish-language channels are bucking the trend. This trend is driven by sport and soaps. One of the larger Spanish-language networks is Univision. For 2013-14, 92% of its 18-to-49 year-old demographic watched programming live compared low 60% figures for the four big English-language broadcasters. Univision's performance was aided it showing the FIFA World Cup from Brazil.
However, the Univision story might indicate only a temporary reprieve for real-time TV. The satellite provider Dish Network was the biggest spender on advertisements on Spanish-language channels! Time will tell if the Spanish-language channels also start to decline. Time will also reveal the extent to which boxing returns to free-to-air TV.