In order of significance:
1. Cost - studies have shown that the average cost of attending an average college game has increased at something like eight times the average increase in wages in the past decade.
2. TV - drastic increase in TV quality and game availability in the past decade. Also, TV times are frequently at inconvenient times to attend. TV has been a double-whammy against attendance.
3. Lack of interest - general interest in sports has decreased dramatically in younger generations.