Science...
Has to do with how the flux effects the heat transfer, the temperature the flux needs to stay useable, the flow of electrons to keep the flux where it belongs, and the intended use of the filler. You are correct that the wire/material will effect which polarity you choose but as a rule of thumb for welding mild steel you use DCEN for E71T-GS flux core wire and DCEP for E70S-6 mig wire. Switching to DCEP on flux wire will usually result in burning through material, undercuts, or burning off the flux causing porosity. Using DCEN on mig wire usually has the opposite effect, no penetration and convex beads. You really would have to learn about all the different types of electrodes, materials, and types of transfer (such as mig spraying, and pulse) to understand why and when to use one polarity over the other.