When the stadium was 59,000 seats, it was damn near impossible to get tickets, regardless of the opponent. The expansion to over 80,000 plus a decade and a half of mediocrity plus watering down the schedule with no-names (Nevada, San Diego State, etc.) plus abject revenue whoring by the administration has taken a big toll on ticket sales. Bigger games still sell out (and even then, you can usually obtain outside the stadium on game day for near face), but the lesser opponents' games are usually widely available. I think the sellout streak is largely BS, because people can't even give away tickets to some games outside the stadium.