A Tokyo Weekender feature in February 2011 on the now-defunct Tokyo Apache provided spot-on insight about the problem:
"Gaining a foothold in the Japanese sports landscape has been tough for the league, which lags far behind baseball, soccer and sumo among other sports. Media coverage is scarce in major outlets and finding games on terrestrial television is a chore. The sport is popular at the grassroots level — the annual high school tournaments draw respectable crowds — but has failed to resonate with fans on the professional level."
Full story: http://www.tokyoweekender.com/2011/02/high-hopes-for-the-bj-league-2/