For lack of killer instinct.
I like the mentality and culture of going for it on 4th and short in reasonable situations. Chip Kelly was aggressive like that at Oregon and I think it helped them develop an identity and confidence that got in the head of their opponents and gave them an edge in converting fourth downs. It could do the same thing for BYU, but not when we are intentionally trying to use 3rd down to set up a manageable 4th down.
4th down should't be viewed as a fall back plan. Call plays in a way to get the first down on downs 1-3. Even if it is 3rd and 8, call a play that is designed to get 9 or more yards. It isn't ideal, but realizing that it happens, develop an offense that has that capability. That can be reliable to pick up the first down. Now, if something happens--player slips and gets just 6 yards, QB is forced to scramble and only gets 7 yards, coverage was good so QB threw it to a check down route and got just 5 yards. Then you are sitting well for 4th down, and the team knows it was an exception that they didn't convert 3rd down, not the result of a play that wasn't going to get the first down without a defensive breakdown. Go ahead, go for it. Be aggressive because the team is already in that mindset. You made an aggressive call the play before to pick up the first down.
This philosophy of planning to use all four downs doesn't promote an explosive offense. It's more of a "3 yards and a cloud of dust" approach. Lack of explosiveness doesn't help you pull away from an opponent and settle the outcome before the final drive. It doesn't take control of a game early and let you dictate how the game is going to be played in the 2nd half. And that is something that has hurt us the last two games.