The pre-Cambrian explosion is linked to the genetic encoding of symmetrical body forms, particularly bilateral symmetry. It just took a while for that to happen. Once it happened there was no turning back.
And how did it happen?
Ddarwin himself had big doubts about that, as well as Stephen Gould. Gould came up with his “punctuated equilibrium “ to try to explain Cambrian - but it proved insufficient.
I used to believe in Darwin’s theory, after reading “Darwin’s Doubt “, I’ve become a strong skeptic.