For what I have seen, it appears likely that in early 2024 the members for Allday Project had been finalized except for the vocalist (now Youngseo). So, Somi knew that a hip-hop focused group with Bailey, Annie, Tarzzan, and Woochan needed a final member to fill the vocalist position at the time.
While Somi was probably not very interested in joining any group, the SB controversy in late 2023 changed things. The controversy was particularly damaging to her due to her mostly casual international fan base (compared to the many other idols suffering from the same cup-induced backlash). Her engagement metrics have fallen significantly as a result, and the comments are far more negative than in the past, hurting Somi's "human-vitamin" brand.
Consequently, in early 2024 it was clear that something drastic needed to take place to revitalize her idol career... like joining a co-ed group with high-profile members.
The high-profiles of most of the other members in ADP would have made it work. Somi wouldn't have completely overshadowed them, avoiding the largest issue people bring up when discussing the possibility of Somi joining a group.
In the past, I have always pushed back against people predicting that Somi will flop due to her long hiatuses. I was proven right in her Dumb Dumb era, then again in her Fast Forward era. But this time is different. Somi's career is in serious trouble. Somi NEEDS a hit (at least in Korea) with either her prerelease "Extra" or her upcoming title single, otherwise she's going to fall into a downward spiral of lower budgets and lower impact (looking at her promo, budgets have already started dropping). The problem is that there is a good chance that neither song will hit, due to the issues Somi is facing.
If her comeback doesn't work out, revitalizing her career by joining a group is no longer an option, and leaving TBL will just make things worse. She will be left without any good options.