no major tv/media partnerships to showcase the entire sport
and promote regularly scheduled events heavily,
cross promote within regular programming,
develop publicity and viewer sentiment for the athletes,
etc.

also individual fights/fighters dont necessarily build the value of one singular banner brand
and vice versa.
like every big game, rules scandal, player arrest, viral candid video, charity drive, etc. all draw interest to the nfl umbrella.
and the fact that someone is an nfl player
is going to draw a lot of eyes and ears to his individual ongoings.
same for the nba (to a slightly lesser degree).

the public has an ongoing back and forth conversation/relationship with these sports and their athletes
24/7 365.

aside from mayweather,
boxer names dont generate hits to websites,
ad revenue,
etc.
so theres really no financial incentive to cover it.