Coming into the 2024 season, Vegas had BYU at 4.5 wins and BYU has beat the spread in 7 of 8 games. There are a lot of reasons for this outlook.

Overall: BYU was 5-7 last year and without further analysis, there wasn't supposed to be a major jump. BYU's recruiting classes didn't show a dramatic uptick in talent. None of the last 4-5 classes showed any competitive advantage against their Big 12 rivals.

On offense, BYU's returning offensive team wasn't supposed to be dramatically improved.
QB - The returning QB was rated by PFF at 69th out of 70 of P4 QBs
RB - LJ Martin was the most promising returning player, but there was nothing proven after that. The loss of Robbins was supposed to be huge.
WR - Roberts was the only receiver with more than 500 yards coming back. Although, there were a lot of returnees, none seemed to be a star.
TE - BYU lost an all-time touchdown leader in Isaac Rex and then promoted a position change WR
OL - 2023 was a very bad year for the Oline. BYU lost its most talented LT1 to the Chiefs in the NFL draft. Another starter graduated.

On defense, there was some improvement in 2023, but BYU lost 5 of its better players (Heckard, Cravens, Tooley, Bywater, Vongphachanh, and Garrett)
DL - The run defense was not very good in 2023 average 4.6 yards per carry given up and had only 11 sacks on the season. Cravens was the biggest loss
LB - Losing Tooley, Bywater, and Vongphachanh meant a full rebuild
DB - Losing Heckard and Garrett made for question marks on who would replace them

Punter - Lost the GOAT

Why was Vegas wrong?
1) The 5-7 record last season was deceptive. The final 2 games could have gone either way. Without the pick six against Oklahoma or the OSU game going the other way could have gotten BYU to 7-8 wins instead of the 5-7 record. The narrative then becomes totally different.
2) BYU hasn't built this team on high profile recruits like most teams. BYU has taken in mostly underrated high school recruits, G5 and FCS (and an occasional P4) transfers who are already game ready. There is also an elite walk on program that allows BYU to play a numbers game to churn out good players with some development. The gamblers are likely very confused that BYU has so many good players with very few 4 star and above recruits. The coaching staff has done an incredible job with player development.
3) The extra consultants have added value. With the addition of the consultants like Gary Andersen (who was a former HC at multiple schools), the game prep and individual one on one coaching has been better this season. More coaches are working with individual units to coach them up particularly on the Dline, which has lead to improvement. The 2023 team and some past teams looked underprepared in some games. 
4) The roster balance is great compared to most seasons. There are no position groups that aren't carrying their weight. This is normally not the case. 
5) Team unity is at an all time high. I'm not sure how/why the team has bonded, but they are playing for each other season. Multiple players say it in interviews that the team wants to win for each other, which generally leads to being "all in" on preparation.
6) ARod fixed his mistakes. ARod's decision to go with a pocket passer to start the 2023 season was a major faux paux especially with a struggling Oline. Moreover, the Oline's biggest issue was the coach had lost the room with some injuries and the Oline didn't gel until late in the season. Those were both fixed in 2024 and caused a massive jump in Oline play and opened up the running game with a mobile threat at QB. Adding the 2 new coaches has also been a huge win for ARod. The new OL coach and new TE coach have been working well together on the run scheme. 
7) Jake Retzlaff had a remarkable offseason. Credit to Retzlaff and ARod for putting in the work. ARod maintaining his redshirt and getting him through the learning curve was super helpful to Jake's development. Jake learned early in his career that he had a turnover addiction that he needed to fix. His failure in 2023 lead him into an offseason that made Jake hungry to prove everyone wrong. It's scary to think how good he could get as a senior if he has another great offseason.
8) The running back room has a lot more talent than their recruiting profiles rankings. Ropati and Martin both look like great backs. Moa looks like a beast behind them and the drop off isn't as steep as expected even after the top 3.
9) The wide receiver room is the deepest talent pool on the team. We thought that before the season, but Lassiter and Roberts as a combo is one of the strongest in school history. I won't be surprised if both play in the NFL. There is a lot of good players behind them.
10) The miracle room is the Oline room. No Kingsley. No problem. The new coach somehow coached up the least effective OL last season in Etienne and made him into arguably the best Olineman. Show the tape from 2023 and 2024 and it doesn't look like the same player. He's the most improved player including Retzlaff. The position group went from bad to very good. There isn't even a noticeable drop off with a 4 year starter injured. Sign this coach up to a long-term contract.
11) The DLine bulked up and they got a lot of extra coaching. They spent the offseason getting bigger and stronger. Gary Andersen (and other consultants) is tutoring them. Batty coming back to be the group leader helped. There are a lot of upperclassmen who are good players now.
12) The linebacker overhaul has been an upgrade over 2023. Taggart showed flashes last year, but he's now the leading tackler on the team. Glasker is blooming into a star. Kelly who came from Weber State appears to have an NFL future. Could all 3 linebackers end up in the NFL?
13) Hill has somehow turned the secondary into the strength of the defense. Robinson probably sleeps in the film room and is more dedicated than anyone in recent memory. The safety room has apparently at least 6 players that Hill thinks could start. Wall and Wakley have emerged as probably earning most of the reps. The young safeties look like they could become stars.

Vegas and outsiders who haven't followed BYU will have a tough time explaining BYU's rise.