Yes, Big Sky attendance has been on a steady decline since Boise State, Idaho and Nevada left the conference in the mid-1990s. (Big Sky average attendance has dropped in nine of the 15 seasons since BSU and Idaho's last year in the league in 1996.) These three programs were replaced by Sac State and Portland State, neither of whom have ever drawn well, and Northern Colorado, who has responded to good teams with increased attendance in the last couple of years -- but still plays in a very small gym. As a result, average attendance last year was 2,900 per game -- compared to 4,400 per game in BSU and Idaho's last BSC seasons.
In addition to losing three established rivals that had good programs when they left the conference, the Big Sky suffers from the same problems as most mid-major schools: over-exposure. You can sit at home in your easy chair and watch college and NBA hoops most any night of the week from the time the NBA starts in late October until the finals conclude in June. And on any given night, with a satellite dish or even a decent cable package, you can watch dozens of college basketball games.
Teams like Weber State, Sacramento State and Northern Colorado also have to compete with NBA franchises in their back yards (And NAU and Idaho State, if you consider a three-hour drive "in your backyard.") And traditional Big Sky basketball schools Montana and Montana State have seen football become the "big dogs" on campus, clearly drawing attention away from the basketball programs.
Finally, Big Sky basketball is probably a below-average product when compared to the BCS conference schools most years. Every now and then a Big Sky school will rise up and win an NCAA tournament game, but one is the limit. You have to go back to Idaho State in 1977 to find the last time a BSC team won more than one game in the NCAA tournament. And you have to go back to Idaho State in 1958 and 59 for the last time a Big Sky team won an NCAA tournament game in back-to-back seasons. As a result, there is no sustainable momentum from one season to the next for any of the league schools.