They simped for the Winnower.

In Short; Because of the Hive

Blame Crota

The Vex had multiple different Factions within Sol before the events of D2. Each faction was exploring/finding possible ways for the vex to survive till the very end of time. To exist beyond everything. The Sol Divisive are special as they seem to be the only Vex faction to link themselves to a paracausal entity, that being the black heart. To protect the rest of the vex collective from possible corruption, the Sol Divisive were separated from the network, unlike the other factions. Atheon seems to be the only Axis Mind we have seen to have control over multiple factions of Vex, that is until the Conductor, who has the Echo of Control, allowing her to manipulate the very radiolaria within the Vex. I do wonder if the Echo of Control could overpower the Sol Divisive as while they are still Vex, radiolaria and all, they had created a connection to the Black Heart, and by extension, to the Witness, meaning that despite still being Causal, they may have some paracausal resistances to corruption outside of their guiding force.

They are a branch of Vex exploring a possibility. They learned they could potentially harness the power and it could be useful for their plans. https://www.ishtar-collective.net/cards/xxxix-open-your-eye-go-into-it#books-of-sorrow But I don’t think they ever harnessed other powers. It was their only paracausal option I believe.

They went a bit further into it last season, but the main reason is that they believed it was the only path to surviving to the end of the universe by allying with a paracausal power. The main objective of Vex is to simulate enough realities to determine how to survive until they are the last race alive, but the Sol Divisive decided to join the Witness rather than fight them. Other Vex factions tend to give them a wide berth like seeing a crazy person on the street instead of interacting with them.