What is inhibiting polystyrene's ability to dissolve in traditional solvents like ethanol or acetone?
Polystyrene is soluble in acetone, MEK and other ketones, but solubility and the speed with which it dissolves depends, in part, on the molecular mass of the polymer.

If the solvent diffuses very slowly into the polystyrene, only the surface will appear to be affected. Applying a bit of energy with heat or mechanical crushing of the plastic greatly speeds the action of the solvent.

For more technical information, see Solubility Parameters: Theory and Application.