I imagine some of it has to do with the development of a “higher” Christology. I don’t think the evidence suggests that Jesus’s immediate followers believed he was resurrected, but that he came to be identified as a divine figure around the 200s. Then, it makes sense to adopt the cross as your symbol - because you are arguing that your guy defied that brutal execution.
The shift was starting around the time of this image I think - pretty solid bit of iconography by the 300s.
I imagine some of it has to do with the development of a “higher” Christology. I don’t think the evidence suggests that Jesus’s immediate followers believed he was resurrected, but that he came to be identified as a divine figure around the 200s. Then, it makes sense to adopt the cross as your symbol - because you are arguing that your guy defied that brutal execution.