Traction factors:
compound,
Tire pressure,
Contact patch size/shape,
wheel/tire diameter/width/weight,
total bike rider weight, I would think Us big boys would stick better LOL
roadsurface,
load and forward/rear weight shift.

I would also think IMO that tire sipes IF of a reasonable size and riding a properly chosen tire for
that bike/rider/roadsurface,,I would think Sipes were a minimal factor compaired to the others,,

what did I miss ?