Probably some rights issues which, as m9 mentioned, might be resolved with UFC partnering with ESPN.
At least for me, I enjoyed the 30 for 30s on stuff from the 80s/90s. I feel like recent stories have been covered pretty well. Bill Simmons shoehorned in a 30 for 30 on the Red Sox 2004 championship in 2010 which felt like it was too fresh compared to the other subjects.
And in a lot of cases, if you don't have some of the principle subjects cooperating, the documentary won't be as good (ie, Sportsnet did a 20 minute documentary on Alex Mogilny's defection but Mogilny declined to be interviewed). ESPN also ran into problems where there were interesting subjects, but really only 10-20 minutes worth of material which they spun off into the 30 for 30 shorts.
UFC/Fox produced its own documentary for their 20th anniversary and that was interesting to see the beginnings of the organization.