#3 WWE doesn't intend to push him to the main event scene
Last year, Braun Strowman won the Men's Money in the Bank Ladder match and cashed in inside Hell in a Cell against Universal Champion Roman Reigns. The match ended in a no-contest after interference from Brock Lesnar. Later on, Strowman lost to Lesnar for the vacant Universal Title at WWE Crown Jewel in Saudi Arabia.
It seems that WWE isn't currently interested in pushing The Monster Among Men to the main event scene. There are rumblings that Vince McMahon - who once wanted to push him as a main event star - now views him as a "walking mascot". This could be another reason why he is no longer part of the MITB ladder match.
#4 Possible heel turn
Braun Strowman turned heel during his feud with Universal Champion Roman Reigns last year. He formed an alliance with Dolph Ziggler and Drew McIntyre to face The Shield. Sadly, after Reigns was diagnosed with leukemia, the company needed a top babyface, so they turned Strowman face once again for his feud with The Beast Brock Lesnar.
With Reigns back to WWE, there is a good chance Strowman is going to embrace his heel persona soon. Actually, his heel turn was what made him a huge star during his run in 2017. So becoming a dominant heel could help re-build Strowman. Perhaps losing his spot in the ladder match can spark this turn and give his character direction for the coming months.
#5 Sami Zayn is a reliable option
It is no big secret that ladder matches involve a lot of pace and agility; even though giants provide some brutality, it is more common to include high-flyers. Sami Zayn may not be flying around like 205 Live stars, but still, he is one of the most thrilling in-ring performers in WWE. At WrestleMania 32, Zayn was quite a few noticeably dangerous spots in the Intercontinental Title Ladder match, for instance.
While Strowman would not hurt the match by being in it, Zayn's style meshes well with the group partaking in the MITB ladder match this year; two big men Baron Corbin and Drew McIntyre are already involved in the match. The inclusion of Zayn very likely means he and RIcochet will provide the high-flying action in the match.
Whatever the reason, we hope Strowman rebounds from this quickly and regains momentum.