I think the biggest issue causing a) players moving on and b) players being reluctant to sign was the lack of a coach for 2025. Once McKellar signed up and started to appoint what looked like a pretty reasonable coaching team, people started to become a lot less reluctant to sign up.
Most of the Tahs who moved on were approaching or had reached 30, Wallaby selection was looking dubious at best, and I would suggest were looking to make a bit of coin from their last few professional years.
The exceptions were Harrison, who probably decided that two years of injury had set him too far back behind the other 10's, and Marky Mark (Nawaqanitawase), who the best I can figure out just wanted to have a crack at league, and probably decided he was young enough to come back to rugby if he wanted to.
As to Pietsch, my best guess (and that's all it is) was that with Suu'ali and Kellaway (Kellaway had announced his return to the Tahs before the Rebels were shut down) signings and at that point Marky Mark (Nawaqanitawase) still on board, he probably didn't see himself getting much footy at the Tahs. Marky Mark (Nawaqanitawase) leaving has probably changed that a little for him.
A lot of this is just my speculation, just trying to apply some logic to it all.