Why are hexa airfoils used on rockets despite having more drag that diamond airfoils?
A rocket stabilized by fins alone needs a certain amount of lift at the rear to ensure that if it is disturbed from a nose-first attitude, it returns and damps out the resulting oscillation.  Adding mass at the rear of the rocket increases the amount of lift (hence fin area) required, as well as increasing the inertia factor that counters damping.

If a diamond fine provides little if any more lift, but weighs more (as it most likely does) than a flat-sided hexagonal section, it's probably a net negative in this compromise.  The added mass makes it harder to damp oscillation, and requires more area, which adds more mass.  Drag, on the other hand, helps a little (since it's near the rear of the vehicle, it tends to pull the tail behind the nose).

End result, for a rocket, it's likely that both on a lift/mass basis, and in consideration of drag, the flat-faced hexagonal fin shape is superior for overall performance.