Nuclear Fusion: What is the cause of transport? (Plasma leakage)

Context: I’ve been reading about The Hairy Ball Theorem which shows that the ideal shape for magnetic confinement fusion has to be a torus. Given that tokamaks use toroidal magnetic fields, I assumed that plasma leakage then wouldn’t be a problem.

What is the cause of plasma leakage?

(If I misunderstood anything, feel free to correct me)